Make the sauce: combine the mayonnaise, ketchup, Worcestershire, Dijon, caramelized onions, sweet relish, vinegar, garlic powder, salt and black pepper. Mix well and refrigerate at least 30 minutes.
Heat a skillet over high heat with a little oil. Place 2-3 tablespoons of ground beef in the center of each tortilla and press into an even layer. Sprinkle with salt.
Cook meat-side down over medium-low heat 4-5 minutes until cooked through, then flip and cook the other side until crispy and golden.
Top each taco with a slice of cheddar, chopped lettuce and caramelized onions. Drizzle with the In-N-Out sauce.