Preheat the oven to 375 degrees F. In a large bowl, combine the ground beef, tomato paste, onions, green pepper, oats, egg, salt, and pepper. Shape the mixture into a loaf and place it in a 9 by 5-inch loaf pan.
In a small bowl, whisk together the ketchup, brown sugar, and mustard. Slather the glaze on top of the meat loaf and bake until the meat loaf is firm and cooked all the way through, about 1 hour. Serve hot.