Mix mayonnaise, ketchup, and mustard in a small bowl and set aside.
Divide ground beef into 4 loose balls without compacting and season with salt and black pepper.
Heat a flat griddle or cast iron pan over high heat until very hot, place beef balls on the surface, and immediately smash flat with a spatula for 5 to 6 seconds.
Cook for 2 to 3 minutes until crispy brown edges form, flip once, add cheese, and cook for 1 minute until internal temperature reaches 160°F / 71°C.
Toast buns cut-side down on the griddle for 1 to 2 minutes until golden.
Spread sauce on bottom bun, add lettuce, place patty with melted cheese, top with diced onion, and close with top bun.