Heat the oven to 350 degrees and lightly grease a loaf pan.
In a large bowl, combine the ground beef, breadcrumbs, egg, milk, diced onion, salt, and black pepper. Mix with your hands just until the ingredients are evenly distributed.
Transfer the mixture to the prepared loaf pan and press it into an even shape.
Spread the ketchup evenly over the top of the loaf.
Bake for 55 to 60 minutes, or until the internal temperature reaches 160 degrees.
Remove the pan from the oven and let the meatloaf rest for 10 minutes before slicing.