Preheat the oven to 400ºF, and get out a casserole-style baking dish.
Using a hand-held grater or food processor, shred the squash and spread it in an even layer in your baking dish.
Chop your sausage into bite-sized pieces, and brown in a large pan over medium-high heat. Remove from the heat, and stir in your chopped veggies (mushrooms, onion, bell pepper) and tomato sauce.
Pour the meat and vegetable mixture over the squash in your baking dish, then top with pepperoni and sprinkle with the minced garlic.
Bake at 400ºF for about 30 minutes, or until the vegetables are tender & the pepperoni and garlic look browned. Your cooking time might vary depending on your oven~ so keep an eye on it for your first try!