Preheat oven to 350 degrees F and line baking sheet with parchment paper.
Wipe mushrooms with moist paper towel or soft brush and remove stems. Dice stems and set aside in a medium-sized mixing bowl. Place mushroom caps on prepared baking sheet.
In blender, add cashews and nutritional yeast flakes and pulse until a fine powder, but not long enough that it creates a nut butter. Scrape corners of blender with spatula to make sure the powder is not sticking to the sides of blender.
Add bell pepper, garlic, sriracha and lemon juice and blend until fully combined and creamy. If it is too thick, a small amount of water can be added. Be cautious not to make it too thin.
In the mixing bowl, fold sauce into the mushroom stems. Spoon mixture into each mushroom cap evenly.
Bake in oven for 20-25 minutes or until filling is firm and golden and mushrooms have released most of their liquid. Serve hot.