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 1/4 cup meat mixture onto each bun half. Cut the remaining cheese slices in half diagonally; place over the meat mixture. Broil 6-8 in. From the heat for 3-4 minutes or until cheese is melted.