Preheat oven to 350 degrees. Place a cooling rack on a large sheet pan. Put the rolls on the cooling rack and toast in the oven for 5 minutes.
While the rolls are toasting, mix together the meat, oats, panko, onion, milk, 4 tablespoons of ketchup, mayonnaise, Worcestershire sauce, garlic powder, egg, and salt and pepper until well combined.
Mix the 2 remaining tablespoons of ketchup and the mustard together in as mall bowl.
Top the toasted rolls with the meat mixture. Be sure to spread the meat to the edges of the rolls so that the bread does not burn.
Spread the ketchup and mustard mixture on top of the meat.
Bake for 20-25 minutes or until the meat is cooked through.
Sprinkle each sandwich with shredded cheese and bake for a few minutes more until the cheese melts.