I have made these with and without the pesto. If you don't have pesto, use a good quality olive oil and balsamic vinegar. A pre-made balsamic "glaze" also tastes delicious, as well.
Preheat oven to 400° Fahrenheit.
If using fresh tomato, slice into 1/4" thick slices and set aside.
If using a ball of mozzarella, also cut into 1/4" thick slices and set aside.
Stir pesto and check for seasoning. Add additional salt and pepper, if needed, to taste, or drizzle in a little olive oil, if the pesto looks too dry.
If using pesto: Top each mushroom cap with a tablespoon of pesto and spread to coat. Add 1-2 slices of fresh or sun-dried tomato, and 1-2 slices of mozzarella or 3 tablespoons shredded mozzarella. Season with salt and pepper to taste.
If not using pesto: Top with 1-2 slices of fresh or sun-dried tomato, and 1-2 slices of mozzarella or 3 tablespoons shredded mozzarella. Drizzle with olive oil and balsamic (if using), and sprinkle with 1/4 teaspoon Italian seasoning.
Bake mushroom caps at 400° Farenheit for 15-20 minutes or until mushrooms have shrunk and cheese is melted and brown.
Serve on top of cooked pasta that has been tossed with remaining pesto sauce, or with garlic-herb cauliflower rice.