Divide the beef into eight equal portions. Press one portion into a thin even layer over one face of each tortilla, reaching the edges; season with the salt and pepper.
Stir the mayonnaise, ketchup, mustard and pickles together. Keep the lettuce and onion ready beside the stove.
Heat a large griddle or two skillets over medium-high heat. Cook the tortillas beef-side down for 3 to 4 minutes, pressing once with a spatula, until browned and the thin beef layer reaches 160°F.
Flip onto the tortilla side, add half a cheese slice to each and cook for 1 to 2 minutes until the tortilla is golden and the cheese melts. Cook in two quick batches if needed.
Top with lettuce, onion and the pickle sauce. Fold gently and serve two tacos per person.