Heat oven to 425°F. Butter a 9-½-inch springform pan.
Cook pasta according to package. Drain and set aside.
In a large skillet, heat the oil over medium-high heat. Add the onion, garlic, and crushed pepper and cook for 1 minute. Then add the sausage and roasted peppers. Cook, breaking the meat into small bits, until cooked through, about 6 minutes. Add the spinach and cook until slightly wilted, about 2 minutes.
Stir in the tomato paste and cook 1 minute, then add the diced tomatoes and salt. Continue to cook, stirring occasionally, and scraping any bits from the pan, until the liquid is mostly evaporated, about 5 minutes. Remove from heat and set aside.
In a large pot (we use the pasta pot), whisk together the milk, eggs, pepper and ¾ teaspoon of salt. Stir in the cheddar, fontina, and 1 cup of Parmesan. Add the sausage mixture and spaghetti. Stir until combined.
Transfer the mixture to the prepared springform pan. Smooth the top with a spatula. Set the pan on a rimmed baking sheet and bake until the edges are golden and bubbling, about 30 minutes.
Remove the pan from the oven and turn on the broiler. Sprinkle the pie with remaining parmesan and fresh basil. Broil until the cheese is golden, about 2 to 3 minutes.
Remove from the oven and run a knife around the inside of the pan. Let the pie rest about 10 minutes then release and remove the sides of the pan. Cut the pie into slices and serve.