Preheat An Outdoor Grill To High Heat, And Lightly Oil Grate. Divide The Ground Beef Into 4 Balls, And Flatten Into Patties Between Sheets Of Waxed Paper. Lay Patties Onto The Grate Of The Prepared Grill. Sprinkle The Tops Of The Burgers With Salt. Grill For About 5 Minutes Per Side, Or To Desired Degree Of Doneness. While The Burgers Are Cooking, Spread Each Top Bun With 1 Teaspoon Of Butter, And Spread Each Bottom Bun With 1 Teaspoon Of Mustard. Place The Buns Onto The Grill For Just A Minute To Toast. Assemble The Burgers In This Order: Bun, Onion, Burger Patty, Cheese, Tomato, Lettuce, A Pinch Of Salt, And Top Bun. Spread Mayonnaise Onto Top Bun If Using.