Preheat the oven to 350 degrees F (175 degrees C).

Mix together ground beef, onion, bread, beaten eggs, milk, bell pepper, salt, sage, and black pepper in a large bowl until well combined. Press into a 9x5-inch loaf pan. Spread ketchup over the top of loaf.

Bake in the preheated oven until no longer pink in the center, about 45 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).