In a small bowl, whisk together mayonnaise, mustard, ketchup, and relish to create the signature sauce; set aside.
Divide the ground beef into four 4-ounce portions and roll into loose balls.
Heat a heavy cast-iron skillet over high heat until lightly smoking.
Place beef balls into the dry skillet and immediately use a heavy spatula or burger press to flatten them into very thin patties.
Season the patties with salt and pepper and sear for 2 minutes until a dark crust forms.
Flip the patties and cook for an additional 1 minute until fully cooked.
Layer shredded lettuce in two large bowls.
Top lettuce with tomatoes, red onions, pickles, and shredded cheese.
Place two smash burger patties into each bowl, breaking them into bite-sized pieces if desired.
Drizzle with the prepared sauce and serve immediately.