Preheat the oven to 425 degrees F.
Prepare the spaghetti squash Cut off both ends so the spaghetti squash is flat. Then cut horizontally. Scoop out the seeds, drizzle with a little oil, and season with salt and pepper. Lie on a baking sheet cut side down. Roast for 15 minutes.
Meanwhile, make your sauce. Heat a large saucepan to medium heat. Add the remaining oil, then toss in the diced onion. Saute for 5-6 minutes, then add the garlic and sausage. Cook until the sausage is no longer pink. Toss in the Italian seasoning, salt, tomatoes, red wine vinegar, and tomato paste. Let simmer for about 10 minutes.
In a small bowl, mix together the ricotta cheese, milk, and mozzarella cheese.
Remove the spaghetti squash after 15 minutes. Once the tomato sauce is ready, transfer the spaghetti squash to a baking dish, cut side up. Fluff up the spaghetti strands with a fork. Sprinkle the bottom with a little parmesan. Now scoop the tomato sauce evenly between the 4 spaghetti squash. Top with a scoop of the ricotta cheese mixture. Sprinkle with some parmesan cheese. Cover with foil and return to the oven.
Cook for about 15 minutes, remove the foil, and cook another 15 minutes until the top is lightly golden. Top with fresh basil and salt to taste. Serve hot!