Begin by preheating your oven to 350 degrees Fahrenheit (175 degrees Celsius). Prepare a 9x5-inch loaf pan by lightly greasing it.

In a large mixing bowl, combine the ground beef, chopped onion, milk, dried bread crumbs, and the large egg. Season this mixture with salt and pepper to your liking, then transfer it into the greased loaf pan.

In a separate small bowl, blend together the ketchup, brown sugar, and prepared mustard until they are thoroughly combined. Pour this mixture over the meatloaf, spreading it evenly across the top.

Place the loaf in the preheated oven and bake until the center is no longer pink, approximately one hour.