Begin by preheating your oven to 350 degrees F (175 degrees C). Lightly oil a 9x5-inch loaf pan to prepare it for the mixture.

In a spacious mixing bowl, blend the ground beef with the chopped onion, milk, and bread crumbs. Crack in the egg and sprinkle generously with salt and pepper, mixing until the ingredients are fully integrated. Once mixed, press this mixture into the prepared loaf pan, creating an even layer.

For the glaze, in a separate small bowl, combine the ketchup, brown sugar, and mustard, stirring until the mixture is smooth and homogenous. Pour this glaze over the top of the meatloaf, ensuring it's evenly distributed.

Place the loaf pan in your preheated oven and bake for approximately 60 minutes, or until the meatloaf is thoroughly cooked through and no longer pink in the center.

Serve this classic meatloaf warm, ideally accompanied by a side of mashed potatoes and steamed vegetables for a complete meal.