Preheat oven to 350 degrees.
Using a damp paper towel or cloth, wipe off mushroom caps. DON’T RINSE OR GET TOO WET!
Using a spoon, scoop out all of the gills from the inside of each cap; brush olive oil on tops and bottoms and place on a baking sheet, top side facing down.
In a small skillet over medium heat, sauté tomato, onion, salt and pepper, and garlic for about 4 minutes; spread mixture on top of mushroom caps and sprinkle with the cheese.
Bake for 3 to 5 minutes. WATCH CAREFULLY TO MAKE SURE ‘PIZZAS’ DON’T BURN!