Heat oil in a large skillet over medium heat. Add onion, peppers, and garlic and sauté for 4 minutes. Add tomatoes and cook for 2 minutes. Season with salt and pepper. Remove from heat and place ingredients in a bowl and cool.
Fold in shredded mozzarella and basil.
Place lightly oil mushrooms on a baking sheet. Fill mushroom caps with a generous amount of mixture. Sprinkle with bread crumbs and parmesan. Place in oven and bake for 20 minutes, or until filling is hot and mushrooms begin to soften. Serve hot.