Remove the stem from the Portobello mushroom. Using a damp paper towel, gently scrub the mushroom to remove any dirt.
Flip the mushroom underside up. With a spoon, scrape away the dark brown fibers underneath the mushroom cap This helps remove the earthy and funky flavor that these fiber produce.
Place the mushrooms on a plate and cover with saran wrap. Microwave for 3 minutes and flip every minute to ensure both sides are evenly cooked.
Carefully remove the mushrooms from the microwave and pat dry with a paper towel. Set aside
In a bowl, combine the Dijon mustard, Sriracha, Worcestershire sauce, soy sauce, smoked paprika, cayenne, cumin, sugar, salt and pepper. Mix until well combined.
Heat a pan on medium low heat and toast the buns with butter. Remove and set aside.
In the same pan, heat 1 tbsp of oil and add the mushrooms. Cook for about 1 minute on each side.
Next, use a brush and lather the glaze on both sides of the mushroom. Cook for about 1-2 minutes on each side or until slighty charred.