Preheat the oven to 375 degrees F (190 degrees C).
Place the mushroom on a baking sheet, and bake for 5 minutes in the preheated oven. Remove from the oven, and spread spaghetti sauce in the cup of the cap. Top with cheese, olives, pepperoni and garlic.
Bake for an additional 20 minutes, or until cheese is melted and golden.