Divide the meat into 4 even balls. Flatten each ball a little to form a rounded, fattish patty about 2 inches thick. Using your thumb, form a divot in the center of the burger (this prevents the burger from "puffing" up into a ball when you cook it). Refrigerate the burgers until ready to cook.
Preheat a grill to medium-high. Sprinkle both sides of the burgers with salt and pepper. Place on the grill, tent with foil and cook about 5 minutes without moving. The burgers should easily release from the grill to turn. Gently flip and cook the burgers to medium rare, 3 to 5 minutes more.
Spread the buns with mayonnaise and toast 1 to 2 minutes on the grill, watching carefully so they don't burn. Serve the burgers on the toasted buns with mustard, tomato, lettuce and onions.