Optional toppings: Lettuce leaves and tomato slices
In a small bowl, mix cheddar cheese, cream cheese, parsley and 1 teaspoon mustard. In another bowl, mix green onions, bread crumbs, ketchup, seasonings and remaining mustard. Add beef; mix lightly but thoroughly.
Shape mixture into 8 thin patties. Spoon cheese mixture onto the center of 4 patties; top with remaining patties, pressing edges firmly to seal.
Grill burgers, covered, over medium heat or broil 4 in. from heat until a thermometer reads 160°, 4-5 minutes on each side. Serve on buns with toppings as desired.