Clean out gills of mushrooms and wipe clean with a damp paper towel.
Preheat oven to 400 degrees F.
Spray mushrooms with avocado oil spray and salt, and bake for 10 minutes on their own.
Meanwhile, make pesto in a food processor or blender using: the basil, garlic, walnuts, olive oil, salt, pepper, and Parmesan/nutritional yeast.
Once the mushrooms are done, add a layer of mozzarella cheese to the base of the mushroom caps.
Top with pesto, tomatoes, olives, and mozzarella balls.
Bake an additional 10 minutes.