Cook Spaghetti: Boil a large pot of salted water and cook 12 oz of spaghetti according to package instructions. Drain and set aside.
Prepare Sauce: In a large skillet, sauté 1 pound of ground beef with 1 diced onion until browned. Add 24 oz of marinara sauce and simmer for 10 minutes.
Mix Ingredients: In a large bowl, combine the cooked spaghetti, meat sauce, and 2 cups of shredded mozzarella cheese.
Layer in Baking Dish: Spread half of the spaghetti mixture in a greased 9x13-inch baking dish. Sprinkle with 1 cup of shredded mozzarella cheese.
Repeat Layering: Add the remaining spaghetti mixture and top with another 1 cup of shredded mozzarella cheese.
Bake: Cover with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10 minutes, until the cheese is bubbly and golden.
Serve: Let it cool for a few minutes before serving. Enjoy your baked spaghetti!