Preheat the oven to 350 degrees and lightly grease a loaf pan or baking dish.
In a large bowl, combine the ground beef, breadcrumbs, egg, milk, onion, salt, and pepper. Mix with your hands until the ingredients are evenly distributed but not overworked.
Press the mixture into the prepared pan and shape it into a loaf.
Bake for 40 minutes.
While the meatloaf bakes, stir together the yellow mustard and brown sugar in a small bowl until smooth.
Remove the meatloaf from the oven and spread the mustard mixture evenly over the top.
Return it to the oven and bake for 15 more minutes, or until the glaze looks set and the internal temperature reaches 160 degrees.
Let the meatloaf rest for 5 minutes before slicing.