Heat oven to 375 degrees. 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 for 20-25 minutes, until cheese is bubbly and mushrooms are soft.