In a large skillet over medium heat, add the ground beef and diced onion, and cook, stirring occasionally, until the meat is no longer pink. Drain any excess grease.
Sprinkle the cornstarch over the cooked meat and stir to coat.
Add the diced tomatoes, garlic, Worestershire sauce, oregano and salt. Stir well to combine and cook for about 5 minutes or until it thickens slightly, stirring occasionally.
Add ½ cup of the shredded cheddar cheese and stir to combine. Cook for another minute or so until the cheese is melted.
Split the hamburger buns in half. Place the split buns on a cookie sheet with the cut side up. Divide the meat among the 8 halves of the buns. Sprinkle the remaining ½ cup of cheddar cheese over the tops.
Set the oven to broil, and broil for about 3-4 minutes or until the cheese is melted and golden. Serve immediately.