In a small bowl, mix together the mayonnaise, ketchup, pickle relish, and mustard. Season with salt and black pepper to taste. Chill until needed.

Divide the ground beef into 8 equal portions (about 2 ounces each). Press one portion of beef directly onto each tortilla, flattening it evenly with your fingers until the tortilla is fully covered. Then cover with parchment paper and use a rolling pin or drinking glass to flatten out the ground beef in a thin layer. It should be at least an inch bigger than the tortilla since it will shrink while cooking. Lightly season with salt and black pepper.

Heat a large skillet or griddle over medium-high heat. Place the tacos, beef side down, onto the hot surface. Depending on your pan size, you may need to work in batches. Press down firmly on the tortilla side using a second skillet, burger press, or another heavy object. Cook for 2–3 minutes, until the beef is crispy on the edges.

Flip them so the tortilla side is down. Top each patty with a slice of cheese and cook for another 1–2 minutes, until the cheese has melted. Remove from the heat.

Top with shredded lettuce, dill pickles, and pickled onions. Drizzle on the sauce.