In a large skillet, cook ground beef and onion over medium-high heat until the beef is browned and onion is tender, about 5-7 minutes. Drain excess grease.
Stir in ketchup, brown sugar, Worcestershire sauce, and mustard into the beef mixture. Season with salt and pepper to taste and simmer for 5 minutes.
Combine the cooked macaroni and 1 cup of cheese into the beef mixture. Mix well.
Transfer the mixture into a greased 9x13 inch baking dish. Sprinkle the remaining cheese on top.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly. Serve hot.
Notes
Feel free to add some extra veggies like bell peppers or mushrooms for added flavor and nutrients.