1Blend mayonnaise, 2 tablespoons relish, capers, 1 tablespoon chopped onion, parsley, and sugar in a blender until smooth. Stir remaining relish and onion into the blended mixture.
2Transfer sauce to a bowl, cover with plastic wrap, and refrigerate at least 2 hours before serving.
Footnotes
Cook's Note:
If you have a problem blending this because it's too thick, add in a teaspoon of milk or cream. Also, use dill relish, not sweet dill relish.