Lightly spray the wells of the cupcake tin with cooking spray and set aside.
Place the ground beef into a mixing bowl, grate the onion into it, and mix to incorporate. Then add the remaining ingredients, except the ketchup. Stir until just combined.
Lightly pack the meat mixture into the wells of a round silicone cupcake or muffin mold. It should make 12 meatballs.
Place the muffin mold in the oven and bake for 18-20 minutes. Baste with ketchup after 15 minutes of cooking and again when they are done.
Remove the meatballs from the mold and top with ketchup. Let cool for a few minutes before serving.