Preheat the oven to 375°F and grease a 9x13-inch baking dish.
Cook the pasta in salted boiling water for 2 minutes less than the package directions. Drain.
Meanwhile, cook the beef and onion in a large skillet over medium-high heat for 6–7 minutes, until browned. Drain excess fat and stir in the garlic.
Add the marinara, diced tomatoes, and Italian seasoning. Simmer 5 minutes. Stir in the cream cheese until melted and season with salt and pepper.
Combine the pasta and sauce and transfer to the baking dish.
Top with mozzarella and cheddar. Bake 20 minutes, until the cheese is melted and bubbly. Broil 1–2 minutes for a golden top, if desired.
Rest 5 minutes before serving.