Preheat the oven to 400°

Place the mushrooms in an oven-proof dish. Remove the stem and drizzle the olive oil over both sides, finishing with the gill side up.

I put the tomato on first so the cheese would spread out easier, but you can put the toppings on in any order you'd like.

On top of the tomato add the cheese, onion, basil, and garlic.

Top that off with the pepperoni.

Place in the oven and bake for about 20 to 25 minutes, until the cheese is melted and the pepperoni is starting to crisp.

Allow to cool for about 5 minutes before eating. That mushroom gets HOT!