Remove mushroom stems and wipe mushroom caps with a damp cloth to remove any dirt. Lightly spray each mushroom cap with nonstick olive oil cooking spray and sprinkle with salt and pepper.
Grill each mushroom 2 minutes per side or until it starts to shrink. Liquid will come out of the mushroom so be sure to pour it off before continuing with the next step.
Spoon 2 tablespoons of pizza sauce into each grilled mushroom cap. Top with cheese and pepperoni. Transfer pizza mushrooms to a baking sheet and broil 1 minute or until cheese is melted. Serve immediately.