Heat oven to 375° F.
Line baking sheet with parchment paper. Coat with olive oil spray.
Using spoon, scrape out dark gills from mushrooms; discard gills.
Place mushrooms stem-side up and top each with 2 tablespoons sauce. Sprinkle each with 2 tablespoons mozzarella cheese and four slices pepperoni or chorizo.
Bake 20-25 minutes, until cheese is bubbly and mushrooms are soft.