Tartar sauce is a cold, creamy condiment built on mayonnaise and studded with finely chopped pickles or gherkins, capers, onion or shallot, and herbs, all sharpened with a squeeze of lemon or a splash of vinegar. Tangy, briny and flecked green, it is the classic accompaniment to fried and battered fish the world over — the sauce that meets fish and chips, fried oysters, crab cakes and fish fingers.