Heat the oven to 350 degrees and lightly grease a loaf pan.
In a large bowl, combine the ground beef, crushed saltine cracker crumbs, milk, egg, chopped onion, salt, and black pepper. Mix with your hands until the ingredients are evenly distributed.
Press the mixture into the prepared loaf pan and smooth the top.
In a small bowl, stir together the ketchup, brown sugar, and mustard until smooth. Spread this mixture evenly over the meatloaf.
Bake for 1 hour or until the internal temperature reaches 160 degrees. Let the meatloaf rest for 10 minutes before slicing.