Heat oil in a large skillet. Add onions and garlic and cook, stirring, until soft, 5 minutes. Remove from the heat and let cool slightly.
In a large bowl, combine beef, pork, cereal, eggs, milk, mustard, salt, and pepper. Blend in the cooled onion mixture.
Spread the mixture into the pan, using a fork to loosen the meat from the sides of the pan. Spread the ketchup over the top. Bake until the meat is cooked through and the center of the loaf registers 160°F on a meat thermometer, 50-60 minutes. Let stand 20 minutes before slicing.