Preheat oven to 350 degrees and line a rimmed baking sheet with parchment paper or aluminum foil.
Spread 1 tablespoon of marinara in each mushroom cap, top with a sprinkling of mozzarella cheese and a healthy dose of pepperoni. Top with more marinara and mozzarella.
Bake for 10-12 minutes or until cheese is melted and bubbly.
Remove and top with parsley. Serve hot.
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Notes
To make these into party appetizers, you can use smaller cremini mushroom caps. Chop pepperoni into smaller pieces and follow the same instructions.