Preheat the oven to 350 degrees F and lightly grease a loaf pan.
In a large bowl, combine the ground beef, breadcrumbs, egg, milk, onion, salt, and pepper. Mix until the ingredients are evenly distributed.
Transfer the mixture to the prepared loaf pan and press it into an even shape.
Bake for 45 minutes.
While the meatloaf bakes, stir together the ketchup, brown sugar, and mustard in a small bowl until smooth.
Remove the meatloaf from the oven and spread the glaze evenly over the top.
Return the pan to the oven and bake for 15 more minutes, until the glaze looks set and the internal temperature reaches 160 degrees F.
Let the meatloaf rest for 10 minutes before slicing.