Preheat the oven to 400F. Grease a 9x13 baking dish.
In a pot of boiling salted water, cook the pasta until al dente, about 10 minutes. Drain and set aside.
In a large sauté pan over high heat, add the oil and onions and cook until soft, about 5 minutes. Add the ground beef and garlic and cook until browned, about 10 minutes, breaking the meat up with a spoon as it cooks.
Add the marinara sauce and bring to a simmer. Season to taste. Add the pasta, 1 ½ cheese and sauce to the prepared baking dish. Toss to mix. Top with remaining cheese.
Bake for 10-15 minutes or until the cheese is melted and bubbly.