Bring a large pot of water to a boil. Preheat the oven to 400ºF. Grease a 9 1/2-inch springform pan using either non-stick spray or butter. Tightly wrap the bottom of the pan with aluminum foil, crimping the foil against the outer edges to tightly seal.
In a large skillet, heat the oil over medium-high heat. Add the onion and garlic and cook for 1 minute. Add the sausage and cook, using the back of a spoon to break into small bits, until cooked through. Stir in the marinara sauce and fresh basil; season to taste with salt and pepper. Remove from the heat and set aside.
Add salt to the boiling water and cook the spaghetti just until al dente, 7 to 8 minutes. Drain the pasta (do not rinse) and return to the pot.
In a medium bowl, whisk together the milk, eggs, pepper, and 1 teaspoon salt. Combine the spaghetti and milk mixture in the pot; add in the sausage mixture, cheddar, mozzarella, and 1 cup of the Parmesan; stir until well combined.
Transfer the pasta mixture to the prepared pan and smooth the top with a spatula. Set the pan on a baking sheet and bake until the edges are golden and bubbling, about 30 minutes. (If it browns too quickly, cover with foil.)