Add meat and diced onions into skillet over medium heat. Cook and break up until there is just a little bit of pink left on meat. Drain grease.
Lower heat to low and add ketchup, mustard and brown sugar. Fold together so meat is well coated. Allow to simmer for 5 minutes then turn heat off and serve. Put a lid on meat that isn't served immediately so it stays warm and continues to marinate.
Serve on buns with sides like a side salad, vegetables or something to add to your bun like crispy tater tots or a scoop of cool coleslaw.