Tartar Sauce

Tartar sauce is a thick white sauce made from mayonnaise and finely chopped pickled cucumber, capers, onions (or chives), and fresh parsley. Chopped hard-boiled eggs, olives, and horseradish are sometimes added, and dijon mustard is often used as an emulsifier. It is frequently used to season fried seafood dishes. It can also be made by mixing commercially produced relish and mayonnaise. Vinegar can be added for a sharper flavor. The sauce is typically of a rough consistency.

http://en.wikipedia.org/wiki/Tartar_sauce
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