Mushroom Onigiri

If you’re a mushroom lover or you’re simply looking for a vegetarian onigiri filling, this recipe is for you! 

Last year, I was on a vegetarian diet for a short couple of months and I noticed how difficult it was to find healthy vegetarian food options aside from salads, smoothie bowls, and falafels. Not that there’s anything wrong with them because they’re all delicious, but the variety of vegetarian food options was severely lacking. 

Even when I’m on a restrictive diet, I find that it’s still important for me to eat foods that I enjoy and onigiri happens to be one of my favorite lunches and snacks to eat. I’ve previously posted onigiri recipes like my salted salmon onigiri and tuna onigiri but I wanted to challenge myself to make a 100% vegetarian onigiri. And that’s how this mushroom onigiri recipe came to be! 🙂

I recommend using an assortment of ‘meatier’ mushrooms for the filling, like shiitake and king oyster mushrooms, but any type would honestly work for this!

Why you’ll love this recipe

If you love onigiri but are struggling to find a vegetarian-friendly version, this mushroom onigiri recipe might just be your new best friend. Here’s why. 

These small but mighty rice balls are compact in shape and portable, making it an easy-packed lunch or snack when you’re on the go. They’re incredibly versatile and great to have for any meal of the day, from breakfast, lunch, dinner, and even a midnight snack!

The mushrooms are coated with a homemade flavorful mentsuyu sauce and then mixed with chili peppers, scallions, and sesame seeds, making this incredibly savory and comforting. With each bite, you get a taste of the savory mushrooms, lightly seasoned rice, and crisp nori, which sounds like perfection (to me at least hehe). 

If you’re trying to cut back on meat or you’re following a plant-based diet, this mushroom onigiri is a must-try!

Mushroom Onigiri Ingredients & Substitutions

Cooked short grain rice: short-grain rice or medium-grain rice is essential for making onigiris properly. They have a higher starch content which is important for helping the rice stick together. I do not recommend using long-grain rice (like jasmine rice or basmati rice) because they aren’t as starchy or sticky. 

Mushrooms: mushrooms are the star of the show so feel free to use an assortment of your favorite types. For this recipe, I used shiitake, shimeji, and king oyster but you can also use portobello, enoki, and button mushrooms. 

Scallions: scallions add freshness and color to the onigiri, but you can also substitute it with spring onions, or chives, or omit them if you prefer without. 

Toasted sesame seeds: sesame seeds provide a nutty flavor and crunchy texture to the onigiri. Be sure to use toasted sesame seeds for a deeper, richer, and nuttier sesame flavor. If you only have raw or white sesame seeds, toast them in a pan over low heat for 2 to 3 minutes or until slightly golden. 

Red hot chili pepper: red hot chili peppers add a slight kick to the filling. You can use any pepper you like, including serrano, jalapeno, or Thai chili peppers. 

Nori (seaweed) sheets: traditional onigiris is served with nori sheets wrapped around the rice ball. In my opinion, an onigiri is incomplete with the nori sheet so I highly recommend using them for this recipe. This is the same nori used to make sushi or kimbap. You can usually find roasted nori sheets at any Asian grocery market or on Amazon. 

Soy sauce: soy sauce is the base and main seasoning for the mentsuyu sauce. I recommend using Japanese soy sauce or Korean soy sauce due to the complexity of flavor but you can also regular soy sauce.

Mirin: mirin adds a distinct sweet taste to the sauce, but you can substitute it with cooking wine or dry sherry. 

Sake: sake can add a hint of taste sweetness or earthiness depending on which type you use, giving the filling a more elevated flavor. You can substitute it with mirin or skip it. 

Dried kelp: dried kelp or kombu elevates the salty and umami flavors of the mentsuyu sauce. You can find dried kelp at your local Asian market or on Amazon. 

Onion: onion balances out the saltiness of the mentsuyu sauce with its sharp flavor and a subtle hint of sweetness. You can use any type of onion, including white, red, shallots, and even scallions. 

How do I prepare Mushroom Onigiri?

Follow along with this easy step-by-step guide to make this delicious mushroom onigiri recipe!

Step 1: Cook the rice

Start by washing the short-grain or medium-grain rice. Cook in a rice cooker or your preferred method of preparation.

Step 2: Make the mentsuyu sauce

Combine soy sauce, mirin, sake, dried kelp, and sliced onion in a pot. Bring to a boil and reduce to medium-low heat.

Next, let it simmer for 7 to 10 minutes, then strain. Set aside while you prepare the onigiri. 

Step 3: Prepare the mushrooms

For this recipe, I’m using shiitake mushrooms and oyster mushrooms. Feel free to use any assortment of mushrooms you like!

Clean off any visible dirt from the mushrooms with a damp paper towel. Dice into small chunks, about 1/2 inch pieces.

Step 4: Cook the mushrooms

Add the chopped mushrooms to a pan and cook for 1 to 2 minutes. Season with black pepper.

Then, add 2 tablespoons of the mentsuyu sauce and stir until the mushrooms are fully coated. Be sure to taste the mushroom and add more sauce for more seasoning.

Step 5: Combine the filling

Transfer the mushrooms to a heatproof bowl and add half the scallions, sliced peppers, and sesame seeds. Mix until they’re thoroughly combined.

Step 6: Prepare the rice

In a bowl with the cooked rice, add the remaining half of the scallions and mix until they’re well combined. If you’re using a mold to shape the onigiris, season the rice with salt now. But if you are shaping the onigiri by hand, don’t season the rice with salt in this step.

Step 7: Shape the onigiri

Now, for the fun part – shaping the onigiris! If you’re using a mold, follow the instructions according to its packaging.

But if you’re shaping by hand, slightly wet your hands then sprinkle a pinch of salt on your hands. Rub your hands together so the salt is evenly dispersed in the palm of your hands. Grab about 1/4 cup of rice and flatten it out on your palms. Then, place 1 to 2 tablespoons of the mushroom mixture in the center. Top with some more rice and press it down so the rice on top sticks to the rice on the bottom. 

Once the rice sticks together, continue to apply pressure and shape the onigiri into a triangle. This may take some practice and a few tries, but practice makes perfect!

Step 8: Add the nori sheet

If serving right away, wrap a sheet of nori on the underside. If serving later, wait to wrap the nori sheet. This will ensure the nori stays nice and crisp. 

Step 9: Serve!

Once you’re done shaping and wrapping all of the onigiris, transfer them to a plate and serve. Enjoy!

Cooking Tips for Mushroom Onigiri

To make the most delicious mushroom onigiri every time, here are a few tips and tricks to keep in mind:

• If you’re cooking fresh rice for this recipe, let it cool down to room temperature so you don’t burn your fingers when shaping. 

• If you’re using a mold to shape the onigiris, cover it with a sheet of cling wrap so the rice doesn’t get stuck to the mold.

• Keep your hands wet when shaping the onigiris to prevent the rice from sticking. I recommend having a small bowl of water while you’re shaping the onigiri. 

• Feel free to adjust the amount of red hot chili peppers in the recipe based on your preferred spice level.
• To speed this recipe up, you can use pre-made mentsuyu sauce instead of making it from scratch. Mentsuyu sauce can be found at your local Asian grocery store or on Amazon.

• If you have any leftover mentsuyu sauce, transfer it to a tight-lid container and store it in the fridge for up to 1 month. This sauce can also be used as marinades for proteins like salmon, vegetable stir-fries, and even for dipping cold soba noodles in. 

Frequently Asked Questions

What is mentsuyu sauce?

Mentsuyu sauce is a concentrated soup base that’s typically diluted for making noodle dishes, such as ramen and udon. It can also be used as a dipping sauce for tempura and cold noodles like soba, but in this case, I’m using it to flavor the mushrooms for the onigiri. It’s delicately sweet, salty, and umami at the same time, making it the perfect sauce for this dish!

You can also buy pre-made mentsuyu sauce at your local Asian grocery store or on Amazon.

Can you use day-old rice for onigiri?

Yes! You can use day-old rice for mushroom onigiri. Just make sure they’re heated and then cooled to room temperature so they’re easy to shape and able to stick together firmly.

Why is there seaweed on onigiri?

Onigiris usually have nori or seaweed sheets wrapped on the bottom to help the rice stay together. They also add a nice crunch and oceany flavor to the dish! While I don’t think an onigiri is complete without the seaweed, you can omit it if you don’t have it on hand. The nori sheets can usually be found at any Asian grocery market or on Amazon. 

Can you make mushroom onigiri in advance?

Yes, you can make this mushroom onigiri a day in advance. Just wrap them in plastic wrap and keep them chilled in the fridge. Wrapping it in plastic wrap will prevent the rice from drying out. Be sure not to wrap the seaweed with the onigiri! This will make the seaweed soggy. The onigiris will have a firmer texture when you serve them the next day, but they will be more flavorful since the rice and filling have had the chance to absorb the seasonings longer. Right before you serve, wrap a nori sheet and enjoy!

Looking for more easy and delicious vegetarian-friendly recipes?

Check out my favorite recipes for more easy and delicious vegetarian dishes:

1. Mushroom Bao – each bite of this mushroom bao will take you on a journey of taste and texture. The tender oyster mushrooms dressed in a delicious savory sauce coupled with pickled cucumbers, spicy Thai chilis, and fresh scallions make this a healthy and irresistible bao.

2. Japchae Dumplings – if you like japchae and crispy fried dumplings, this recipe is for you! Soft and chewy interior with a crispy golden exterior – what more can you want from a dumpling? Although they taste fantastic on their own, dipping them in my 5-minute Korean pancake dipping sauce will make them simply unforgettable!

3. Air Fryer Salt and Pepper Tofu – if you love salt and pepper dishes but want to cut back on grease, this air fryer version is a healthier vegetarian alternative. The addition of sauteed garlic and scallions makes the dish even more irresistible, while the green pepper and Thai chili peppers instantly turn up the heat

4. Eggplant Katsu – crispy eggplant katsu is an amazing ‘meatless’ alternative to the popular tonkatsu dish in Japanese cuisine. The extra crunchy exterior with the soft tender interior is loaded with flavor in every bite. It’s light and flavorful, and you can even serve it with my curry rice recipe.

5. Taiwanese Sesame Noodles (Ma Jiang Mian) – these sesame noodles is one of those quick 10-minute meals that you can make any day of the week. It only takes 10 minutes to prepare, and it’s packed with noodles and a savory sauce!

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