n a small bowl, combine the mayonnaise, ketchup, and mustard. Stir to mix. Set aside. Shape the beef into a patty that is about 1/2" wider than the bun.
Preheat a medium nonstick skillet or grill rack on high heat.Place the patty on the pan or grill. Cook for 1 to 2 minutes per side, or until desired doneness is reached. (Do not smash the burger with the spatula.) About 30 seconds before the burger is cooked, top with the cheese, if desired.
Place the bun halves, cut sides down, on the pan or grill. Cook for about 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.