Preheat your oven to 180°C (356°F). In a large bowl, combine the ground beef, chopped onion, breadcrumbs, milk, egg, tomato ketchup, salt, and pepper. Mix thoroughly until all ingredients are well incorporated. Shape the mixture into a loaf and place it on a baking tray lined with parchment paper. Bake in the preheated oven for 50-60 minutes, or until the internal temperature reaches 70°C (160°F). Let it rest for 10 minutes before slicing. Serve hot with your favorite sides.