Mix mayonnaise, ketchup, and mustard in a small bowl to make the burger sauce.
Preheat a Blackstone griddle or large cast iron skillet over high heat until very hot.
Divide the ground beef into 4 loose balls without compacting, place on the hot griddle, immediately smash flat with a sturdy spatula, season with salt and pepper, and cook for 2 minutes until crispy edges form.
Flip once, top each patty with a cheese slice, and cook for 1 to 2 minutes until the internal temperature reaches 160°F / 71°C.
Butter the cut sides of the buns and toast them on the griddle for 1 to 2 minutes until golden.
Spread sauce on the bottom bun, add lettuce, place the patty with melted cheese, top with diced onion, and close with the top bun.