1Preheat oven to 425 degrees F (220 degrees C). Line 24 muffin cups with paper cups and lightly coat with cooking spray.
2Heat a large skillet over medium-high heat. Cook and stir beef and onion in the hot skillet until the beef is completely browned and crumbly, 5 to 7 minutes; drain and discard grease.
3Mix flour, sugar, baking powder, and salt together in a large bowl. Stir ketchup, milk, butter, eggs, and mustard together in a separate bowl; add to flour mixture and stir until pink in color. Mix ground beef mixture into the flour mixture; spoon into prepared muffin cups to completely filled.
4Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 40 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.