In a large bowl, whisk milk, egg, and salt. Add cheese, oats, onion, and beef.
Mix by hand. Form into 10 small meatloaves. Place meatloaves in a lightly greased 13"x9" baking pan.
In a small bowl, combine catsup, brown sugar, and mustard (if desired). Spoon sauce evenly over meatloaves.
Cover with aluminum foil. Bake at 350 degrees for 30-45 minutes, until juices run clear.