1 tsp. Yellow mustard
1 tbsp. Packed brown sugar
2/3 cup Ketchup
3 Cheeses, cut in half crosswise
1 pound Lean ground beef
1/2 Medium onion, diced
1/4 tsp. Black pepper
1 tsp. Kosher salt
1/2 tsp. Italian seasoning
1/2 cup Quick cooking oats
3/4 cup Milk
1 Large egg