1. Preheat oven to 375 degrees. Cut each spaghetti squash in half. Scoop out seeds and discard. Place each half face down on a greased baking sheet. Bake for 45 minutes or until squash is fork tender. Use a fork to gently pull apart the strands; place strands in a large bowl and set aside. Do not discard rinds.
2. In a saucepan, cook Italian sausage, onion, and garlic over medium heat. When sausage is cooked throughout, drain grease if necessary. Add spaghetti sauce; stir and heat for 3-5 minutes or until sauce is warmed.
3. Add warm sauce to spaghetti squash and thoroughly combine. (Add 1 cup of sauce at a time so you can create a squash/sauce ratio that you like.) Stuff each rind with squash. Top stuffed rinds with mozzarella cheese.
4. Turn oven to broil. Broil for 2-3 minutes or until cheese has melted and is browned. Garnish with fresh parsley.
Dairy-free: Omit cheese. Do not broil.
Paleo: Omit cheese. Do not broil. Use paleo-friendly sauce.
Vegan: Omit cheese. Omit Italian sausage. Sauté onion and garlic in olive oil before adding spaghetti sauce.
Vegetarian: Omit Italian sausage. Sauté onion and garlic in olive oil before adding spaghetti sauce.