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