In a small bowl, mix the mayonnaise, ketchup, and mustard, stir well and set aside.
Shape the beef into a patty that is about 1/2-inch wider than the bun.
Preheat a medium non-stick skillet or grill rack on high heat. Place the patty on the pan or grill. Cook for 1-2 minutes on each side, or until desired doneness. (Do not smash the burger with the spatula).
About 30 seconds before the burger is fully cooked, top with cheese if desired.
Place the bun halves, cut sides down, onto the pan or grill. Cook for approximately 45 seconds, or until toasted.
Place the bun bottom on a serving plate. Top with the patty, lettuce, onion and pickles. Spread the reserved sauce evenly over the inside of the bun top. Flip onto the burger. Serve immediately.