Preheat the oven to 400F

Wash the mushrooms well. Remove the stems from the mushrooms by twisting them off and use a spoon to gently scoop out the gills from the underside of the mushroom caps. Rub the olive oil around the inside and outside of the mushroom cap and place on the pizza pan

Divide the tomato sauce between the 4 mushroom caps, spreading it around evenly. Sprinkle in the diced mushrooms, diced tomatoes, and any other veggies or cooked meat you want to add to your "pizza"

Divide the grated mozzarella between the 4 mushroom caps, then garnish with chopped or torn basil leaves and balsamic reduction, if using

Bake the mushroom pizzas for 15 minutes, or until the cheese is melted and starting to brown