Preheat a skillet or griddle over medium-high heat.
Divide the ground beef into four equal portions.
Form each portion into a loose ball.
Place one ball on the skillet and smash it down with a spatula.
Season with salt and pepper and cook for 3-4 minutes until browned.
Flip the patty, add cheese, and cook for another 1-2 minutes.
Warm the tortillas in a separate pan for 30 seconds on each side.
Assemble the tacos with a patty on each tortilla.
Add pickles, onions, ketchup, and mustard.
Fold the tortillas and serve.