In a small bowl, combine beef, ketchup, mustard and salt, mixing lightly but thoroughly. Cut onion into 1/2-in.
In a shallow bowl, mix flour and garlic powder. Place bread crumbs and eggs in separate shallow bowls.
In an electric skillet or deep fryer, heat oil to 350°. Fry onion rings, a few at a time, until golden brown and a thermometer inserted into beef reads.