Divide ground beef into 8 equal portions and form into loose balls.

In a small bowl, whisk together mayonnaise, mustard, ketchup, and relish to create the burger sauce.

Preheat a large cast-iron griddle or skillet over medium-high heat until lightly smoking.

Place a beef ball on the griddle and immediately top with a flour tortilla.

Use a heavy spatula or burger press to smash the meat through the tortilla into a thin layer that covers the diameter of the tortilla.

Cook for 2 to 3 minutes until a dark brown crust forms on the beef.

Carefully flip the taco so the tortilla side is down and the beef is facing up.

Immediately place a slice of American cheese on the beef and cook for 1 minute until the cheese melts and the tortilla is toasted.

Remove from heat and top with diced onions, lettuce, pickles, and the prepared sauce before folding.