For the version in the picture, I mixed about 1 tablespoon of mustard, 2 tablespoons of mayonnaise, and 3 tablespoons of balsamic vinegar. Experiment with proportions for the taste and consistency you are looking for.
I hate both mustard and mayonnaise, so it is an act of love to make this for my husband. I do it to keep him from going to the store and buying bottles of dressing that are filled with preservatives and inferior ingredients. He said it tastes a little like Burger King’s “special sauce”. On the Fitness Channel I saw another version of this, which replaces the mayonnaise with plain nonfat yogurt.