In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. , In a small bowl, combine the flour, sugar, baking powder and salt. In another bowl, combine the ketchup, milk, butter, eggs and mustard; stir into the dry ingredients just until moistened. Fold in the beef mixture and cheese. , Fill greased miniature muffin cups three-fourths full. Bake at 425° until a toothpick comes out clean, 15-18 minutes. Cool for 5 minutes before removing from pans to wire racks. Serve warm. Refrigerate leftovers.