McDonald's® Big Mac Sauce® ("Special Sauce")

If you like Big Macs, it's probably because of that tasty
 "secret" spread that is plopped onto both decks of the world's
 most popular double-decker hamburger. So what's so special
about this sauce? After all, it's basically just thousand island
 dressing, right? Pretty much. But this sauce has a bit more sweet
 pickle relish in it than a typical thousand island salad slather.
 Also, I found that this clone comes close to the original with the
 inclusion of French dressing. It's an important ingredient -
 ketchup just won't do it. That, along with a sweet & sour flavor
 that comes from vinegar and sugar, makes this sauce go well on 
any of your home burger creations, whether they're Big Mac clones
 or not. This is the closest "special sauce" clone you'll find 
anywhere.

1/2 cup mayonnaise
2 tablespoons French dressing
4 teaspoons sweet pickle relish
1 tablespoon finely minced white onion
1 teaspoon white vinegar
1 teaspoon sugar
1/8 teaspoon salt

1. Combine all of the ingredients in a small bowl. Stir well.
2. Place sauce in a covered container and refrigerate for several 
hours, or overnight, so that the flavors blend. Stir the sauce a 
couple of times as it chills.
Makes about 3/4 cup.