We all know that McDonald's Big Mac has some secret sauce that makes it the Big Mac as opposed to simply a double decker cheeseburger. Because the sauce is a pale peachy pink color with green flecks that resemble pickle relish, many people assume that the secret sauce is just Thousand Island dressing. However, the sauce is not Thousand Island dressing - the sauce is thicker, sweeter, and has a slightly different taste.
Though the list of actual ingredients is long with things like "xanthan gum," I found a recipe in a book called Fast Food Fix that claims that it almost exactly replicates the original sauce. The recipe even uses low-fat mayo, so I guess when you're making Big Macs at home (why you would do this, I have no idea), you can use this recipe for the secret sauce and also feel less guilty:
In a small bowl, combine 1/3 c. low-fat mayo, 2 tsp dill pickle relish, 2 tsp. ketchup, 2 tsp. sugar, and 2 tsp. yellow mustard. (The mustard is what make it definitely NOT Thousand Island dressing.) Cover and refrigerate for up to one month. Stir in 1 Tbsp + 1 tsp. finely chopped white onion right before serving.