In a small bowl, mix together mayonnaise, ketchup, and pickle relish until well combined. Set aside in the refrigerator.

Divide ground beef into 8 equal portions (about 2 oz each) and loosely form into balls. Season generously with salt and pepper.

Heat a large cast iron skillet or griddle over medium-high heat until very hot, about 3-4 minutes.

Place beef balls on the hot skillet and immediately smash flat using a sturdy spatula or burger press until very thin. Cook for 2-3 minutes until crispy and browned on the bottom.

Flip the patties, top each with half a slice of cheese, and cook for another 1-2 minutes until cheese melts and the second side is crispy.

Warm flour tortillas in the same skillet for about 30 seconds per side until soft and pliable.

Place one smash patty in each tortilla, then top with special sauce, shredded lettuce, diced tomatoes, and diced onions. Serve immediately.