Combine soy sauce, mirin, sake, dried kelp, and sliced onion in a pot. Bring to a boil and reduce to medium-low heat. Simmer for 7 to 10 minutes then strain. The sauce can be stored in a tight-lid container for up to 1 month in the fridge.
In a pan, add the chopped mushrooms and allow this to cook down for 1-2 minutes. Add 2 tbsp of the mentsuyu sauce and stir until mushrooms are fully coated. Taste the mushrooms and add more sauce if needed.
Transfer the mushrooms to a heatproof bowl and add half the scallions, sliced peppers, and sesame seeds. Mix until thoroughly combined.
In a bowl with the cooked rice, add the remaining half of the scallions and mix until combined. If you are using a mold to shape the onigiri's, season the rice with salt now. If you are shaping the onigiri by hand, do not season the rice with salt in this step.
Let's shape the onigiri! If using a mold, follow the instructions according to the package.
If you're shaping by hand, wet both hands and using two fingers, dip it in the bowl with salt. Rub the salt into the palm of your hands until evenly coated. place a sprinkle of salt on your hands and then grab about 1/4 cup of rice and flatten it out on your palms. Place 2 tbsps 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. Using your palms, apply pressure and shape the onigiri into a triangle. This may take some practice and a few tries.
After the onigiri is shaped, wrap a sheet of nori on the underside of the onigiri.
Serve and enjoy!
*Onigiri's can be made in advance and can be eaten cold too! To serve the next day, wrap the onigiri in plastic wrap and place in the refrigerator. The plastic wrap will keep the moisture in and prevent the rice from drying out*
