Preheat oven to 375°F. Line a baking sheet with parchment paper.
Gently wash mushrooms and remove stems. Put stems in a small bowl and place mushroom caps on prepared baking sheet, stem side down.
Place caps in preheated oven and bake 10 minutes or until excess water has leaked out of the mushrooms but they have not yet browned.
Remove caps from oven (keep oven on) and place them, top side down, onto paper towels to cool slightly.
To make filling, finely dice mushroom stems into small pieces and place in a bowl. Add diced pepper, onions, toasted bread crumbs or tortilla chips, nutritional yeast, olive oil, cumin, salt, pepper, dried mustard and parsley and stir to combine ingredients.
Spoon dollops of filling into cooked caps, pressing to pack filling into each cap and adding more filling to mound.
Reline the baking sheet with fresh parchment paper. Place filled caps onto prepared baking sheet. Place in preheated oven and bake 10 minutes or until mushrooms are lightly browned and filling is crispy. (Watch to be sure caps do not overcook.)
Serve warm.
Each mushroom cap contains: 21 calories, 1g total fat, 0g saturated fat, 0g trans fat, 0mg cholesterol, 65mg sodium, 2g carbohydrate, 1g fiber, 1g sugars, 1g protein, 1Est GL.