Heat the oven to 350 degrees and lightly grease a loaf pan.
In a large bowl, combine the ground beef, breadcrumbs, eggs, diced onion, milk, salt, and black pepper. Mix with your hands until the ingredients are evenly distributed but do not overwork the meat.
Press the mixture into the prepared loaf pan and smooth the top.
Stir the brown sugar and yellow mustard together in a small bowl until smooth.
Spread half the glaze over the top of the loaf. Bake for 45 minutes.
Remove the pan from the oven, spread the remaining glaze over the meatloaf, and return it to the oven for another 15 minutes.
Let the loaf rest for 10 minutes before slicing.