It is mayonaise sauce with minced pickle, capers and onion added.

Put 2 raw yolks into a pint bowl, add a tablespoon of salad oil and stir together with an egg beater, add more oil and continue stirring, throw in 1/2 tea-spoon of salt and it will become thick almost immediately; then add a teaspoon of vinegar, then 2 of oil and continue until you have enough for the purpose constantly; stirring the sauce, adding oil twice and vinegar once alternately and always in very small portions, and at the finish or when you have near a cupful, squeeze in the juice of half a lemon. Mustard and cayenne may be added if wished, but are not essential ingredients of mayonaise sauce.

Mince a few capers and piece of green pickle and a young onion or two extremely fine, drain the mince on a napkin, stir it into the mayonaise and you have tartar sauce. Serve cold in individual sauce dishes or large butter chips.