Par-cook squash: Pierce spaghetti squash with a fork all around the outside of the squash. Place on a microwave-safe plate and microwave for 3 minutes. Let cool for five minutes.
Prep the equipment: Preheat the oven to 400 F. Line a large sheet pan with parchment paper and set aside.
Bake squash: With a sharp chef's knife, cut the spaghetti squash in half. Scoop out the seeds. Rub flesh sides with a light drizzle of olive oil and rub with garlic cloves. Bake for 20-25 minutes, or until squash can easily be shredded off with a fork.
Shred the squash noodles: Let squash cool for 10 minutes and shred the insides and set aside in a separate bowl. Should yield about 4-5 cups of shredded squash.
Cook the sausage: While the squash is baking, begin to prepare the Italian sausage marinara mixture. In a large saute pan, add olive oil and onion and cook on medium heat for 2-3 minutes, or until the onion is translucent. Add in the ground sausage and break up with a wooden spoon. Cook onions and sausage for 8-10 minutes, or until sausage is cooked through. Drain any excess grease out of the pan.
Finish the meat sauce: Add the marinara sauce into the Italian sausage and the shredded insides of the spaghetti squash. Mix sausage, spaghetti squash and marinara sauce together and bring mixture to a light simmer. Cook mixture for 5-8 minutes, or until the squash is tender.
Fill the squash: Arrange the hollow spaghetti squashes on a large baking sheet and adjust the oven to broil. Spoon mixture evenly into the hollowed-out spaghetti squash.
Top with cheese: Dollop the tops of the squashes with ricotta cheese and sprinkle evenly with mozzarella and parmesan cheese.
Broil: Broil for 5-10 minutes, or until the top is bubbly and brown.
Finish and serve: Garnish with fresh basil and enjoy!