Preheat the oven to 375°F (190°C) and spray a baking sheet with cooking spray.
Clean the mushrooms and remove the stems. Chop the stems finely and place the mushroom caps on the prepared baking sheet.
Melt the butter in a medium skillet over medium heat. Add the mushroom stems, onion, and garlic, and cook until softened and fragrant. Remove from heat and stir in the crushed crackers, pepperoni, parmesan cheese, and parsley.
Spoon the filling into the mushroom caps, pressing it down gently with your fingers to pack the mushroom caps full. Bake for 15-20 minutes, or until the mushrooms are tender and the filling is lightly browned.
Notes
STORE leftover Stuffed Mushrooms in an airtight container in the refrigerator for up to 3 days. To FREEZE, arrange them in a single layer on a baking sheet, put them in the fridge until solid, then transfer to a freezer-safe container for up to 2 months.To REHEAT, bake in a preheated oven at 350°F (175°C) for 10-15 minutes until heated through.