Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
Step 2: Cook the pasta according to package directions until al dente. Drain well and set aside.
Step 3: In a large skillet or Dutch oven, brown the ground beef and chopped onion over medium-high heat. Break up the beef with a spoon as it cooks. Once the beef is no longer pink and the onion is softened, drain any excess grease.
Step 4: Add the minced garlic to the skillet and cook for another minute until fragrant.
Step 5: Stir in the Alfredo sauce, milk, and Italian seasoning. Bring the mixture to a gentle simmer, then reduce heat to low and let it cook for 2-3 minutes, allowing the flavors to meld. Season with salt and black pepper to taste.
Step 6: Remove the skillet from the heat and stir in 1 cup of the mozzarella cheese and 1/2 cup of the Parmesan cheese until melted and fully incorporated into the sauce.
Step 7: Add the cooked and drained pasta to the skillet with the beef and Alfredo sauce. Toss gently until the pasta is evenly coated.
Step 8: Pour the entire pasta and beef mixture into the prepared 9×13 inch baking dish. Spread it out evenly.
Step 9: Sprinkle the remaining 1 cup of mozzarella cheese and 1/4 cup of Parmesan cheese over the top of the casserole.
Step 10: Bake for 20-25 minutes, or until the casserole is bubbling around the edges and the cheese on top is melted and lightly golden brown. For an extra golden crust, you can broil for the last 1-2 minutes, watching carefully to prevent burning.
Step 11: Remove from the oven and let rest for 5-10 minutes before serving. This allows the sauce to set slightly, making for easier serving. Garnish with fresh parsley, if desired.