Step 1: Cook the pasta according to package directions until al dente. Drain and set aside.
Step 2: In a large skillet or Dutch oven, heat the olive oil over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until fully browned. Drain any excess grease. Stir in the diced onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for 1 minute more until fragrant. Season the beef with garlic powder, salt, and pepper. Remove the beef mixture from the skillet and set aside.
Step 3: In the same skillet, melt the butter over medium heat. Whisk in the flour and cook for 1 minute to create a roux.
Step 4: Slowly pour in the heavy cream, whisking continuously to prevent lumps. Bring to a gentle simmer, then reduce heat to low.
Step 5: Stir in 1 cup of freshly grated Parmesan cheese and the nutmeg until the sauce is smooth and creamy. Season with salt and black pepper to taste. If the sauce is too thick, add a splash more cream or milk.
Step 6: Add the cooked ground beef mixture and the drained pasta to the Alfredo sauce. Stir gently until everything is well combined and the pasta is evenly coated.
Step 7: Pour the mixture into a 9x13 inch baking dish, spreading it evenly.
Step 8: Sprinkle the shredded mozzarella cheese generously over the top.
Step 9: Cover loosely with foil and bake in a preheated oven at 375°F (190°C) for 20 minutes.
Step 10: Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and lightly golden brown. For a darker crust, you can broil for 2-3 minutes, watching carefully to prevent burning.
Step 11: Let the Ground Beef Alfredo Bake rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.