1In a bowl, mix mayonnaise, ketchup, mustard, sweet pickle relish, vinegar, sugar, salt, and black pepper to make the Big Mac sauce. Set aside.
2Finely chop the onion, lettuce, and tomato.
3Heat a skillet over medium-high heat. Add ground beef and chopped onion. Cook until beef is browned and onion is soft, breaking up the meat as it cooks.
4Add cheddar cheese to the beef mixture and stir until melted and combined.
5Warm taco shells according to package instructions.
6Fill each taco shell with the chopped cheese mixture.
7Top with lettuce, tomato, and a generous drizzle of Big Mac sauce.