Prep. Preheat oven to 350F and line a baking sheet with parchment paper.
Spaghetti squash. Slice the spaghetti squash (1) in half, and scoop out seeds and fiber with a spoon. Place cut side down on the prepared baking sheet, and bake in the preheated oven for 1 hour, or until the flesh is fork-tender. Set aside, and allow squash to cool for 10 minutes before shredding with a fork into spaghetti. Place cooked spaghetti squash in clean kitchen towels, and ring out any additional liquid.
Sauce. Meanwhile, in a large skillet, heat olive oil (2 tablespoons) over medium heat. Add the onion (1 cup) and saute until softened, about 5 minutes. Add the garlic (4 cloves) and cook until fragrant, about 2 minutes. Add sausage (1 pound), and crumble into small pieces using the back of your spoon as it browns. Cook until no longer pink, then remove pan from heat. Push the meat and veggies to one side of the pan, then tilt the pan to the opposite side so the grease pools. Use tongs and paper towels to sop up extra grease. Add marinara and shredded spaghetti squash to the skillet and stir to combine. Set skillet aside.
Cheese combo. In a medium bowl, combine cottage or ricotta cheese (8 ounces), cream cheese (8 ounces), sour cream (1/4 cup), 1 cup of mozzarella, and Italian seasoning (1 tablespoon).
Assemble. Layer half the spaghetti squash along the bottom of the baking dish, followed by the entire bowl of cheese mixture, then the rest of the spaghetti squash and sauce (2, 24 ouce jars). Top with the remaining 2 cups of mozzarella.
Bake + enjoy! Bake casserole in preheated oven for 35-45 minutes, or until it's heated through and the cheese in melted. Serve with fresh basil slices and enjoy!