I just saw a recipe on Allrecipes.com and don't know why I didn't think of this before. You could add any other toppings you like. This sounds soooo good.

1 teaspoon extra virgin olive oil
1 clove garlic, diced
6 ounces portobello mushroom caps, cleaned
Pinch salt
Pinch freshly ground black pepper
12 ounces mozzarella cheese, sliced or shredded
10 fresh basil leaves
2 fresh tomatoes, sliced, roasted, or grilled
Oregano leaves (optional)

Combine the oil and garlic in a small bowl and rub the mushroom caps on all sides with the mixture. Place the caps, top side down, in a circle on an oiled baking sheet. Season with the salt and pepper. Arrange the cheese, basil, and tomato slices alternately in a circle on top of the mushrooms. Sprinkle with the oregano, if using. Bake at 450 degrees F until the cheese melts, about 3 minutes.