Preheat oven to 375°F. In a large oven-safe skillet or Dutch oven, brown the beef over medium-high heat until no longer pink, about 5 minutes. Add onion and half the garlic and cook 3 to 4 minutes. Stir in marinara sauce and Italian seasoning, season with salt and pepper, and simmer until slightly thickened.

In a separate saucepan, melt butter over medium heat. Add remaining garlic and cook until fragrant, 30 seconds. Whisk in heavy cream and bring to a gentle simmer. Cook 2 minutes, then remove from heat and stir in ¾ cup Parmesan until smooth.

Cook pasta in salted boiling water until just al dente, about 1 minute less than package directions. Drain and return to the pot.

Pour Alfredo over the pasta and toss to coat. Spread half into a greased 9x13 dish. Spoon meat sauce on top, then cover with remaining pasta. Top with mozzarella and reserved ¼ cup Parmesan.

Bake uncovered 25 to 30 minutes until bubbly and golden on the edges. Rest 5 minutes, then serve hot.