Make the Meat Sauce: Cook the onion and bell pepper in olive oil until softened. Add garlic and cook briefly, then brown the ground beef. Drain most of the grease, leaving a little in the pan. Stir in the tomato paste and cook for 2–3 minutes until darkened and caramelized. Add the wine or broth, scraping up any browned bits. Stir in the seasonings and spaghetti sauce, then simmer 10–15 minutes until thickened. Finish with sugar or grated carrot and balsamic vinegar.

Cook the Pasta: Boil the spaghetti in salted water until just al dente, about 2 minutes less than package directions. Reserve 1/4 cup pasta water, then drain. Toss the pasta with butter and reserved pasta water until coated.

Make the Alfredo Sauce: Melt butter in a saucepan and cook the garlic until fragrant. Whisk in the flour and cook for 1 minute. Slowly whisk in the cream and milk, then simmer until slightly thickened. Reduce heat to low and stir in the Parmesan gradually until melted and smooth. Stir in cream cheese if using, then season with nutmeg and salt.

Combine Pasta and Alfredo: Pour the Alfredo sauce over the spaghetti and toss until evenly coated. Fold in 1 cup of mozzarella cheese.

Assemble the Casserole: Preheat the oven to 350°F and grease a 9x13-inch baking dish. Spread the Alfredo spaghetti evenly into the dish, then top with the meat sauce. Sprinkle with the remaining mozzarella and provolone or fontina cheese.

Bake and Serve: Cover with foil and bake for 20 minutes. Remove the foil and bake another 10 minutes until the cheese is melted and bubbly. Broil for 2–3 minutes if desired for extra color. Let the casserole rest for 5–10 minutes before serving.