Preheat oven to 425 degrees. Spray a 9 inch pie dish with cooking spray.
In a large skillet, sauté squash, onion and garlic in a few tablespoons of water, until squash is just soft and onions are becoming translucent. Let cool.
Meanwhile, in a large bowl, combine milk, salt, pepper, eggs, bacon, and parmesan cheese. (Leave out the mozzarella and additional squash for topping). Whisk everything together until filling is smooth and everything is well combined. Add sautéed squash, onions, and garlic.
Pour quiche filling in prepared pie pan. Bake for 30 minutes, remove from oven, and top with additional squash and mozzarella. Return to oven and bake an additional 10-15 minutes, or until cheese is golden brown and quiche is set. Serve warm!