In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain.
Sprinkle with cornstarch; stir until blended. Stir in the tomatoes, oregano, salt and onion salt. Cook, uncovered, for 5 minutes or until slightly thickened.
Add six cheese slices; cook and stir until cheese is melted and blended.
Place hamburger buns cut side up on an ungreased baking sheet; spoon about 1/4 cup meat mixture onto each bun half.
Cut remaining cheese slices in half diagonally; place over meat mixture. Broil 6-8 in. from the heat for 3-4 minutes or until cheese is melted.