Heat 2 tbs extra virgin olive oil over medium heat in a large skillet. Add onion and cook for 4-5 minutes. Add garlic and cook for 30 seconds, stirring continuously.
Meanwhile, heat a grill pan over medium heat. Brush the remaining 1 tbs extra virgin olive oil over tops of the mushroom caps, and place mushrooms upside down on the grill pan (the tops of the mushrooms should be facing you). Cook for 5 minutes. Remove from heat and set aside.
Add mushrooms, zucchini, tomato, oregano, red pepper and salt and cook for 10 minutes, stirring every now and then. Remove from heat and set aside.
Turn mushroom caps over. Fill each cap with the veggie mixture and top evenly with freshly shredded mozzarella.
Add to grill pan and cover with a piece of aluminum foil. Cook for 3-5 minutes, or until cheese has melted.
