Remoulade Sauce

Mash very fine the yolks of three hard boiled eggs and when smooth add half a teaspoon of French mustard

From 1922:

Mash very fine the yolks of three hard boiled eggs and when smooth add half a teaspoon of French mustard, one teaspoon of tarragon vinegar and a seasoning of salt and paprika.
Mix smooth and add a little at a time three tablespoons of good olive oil, then work in the yolk of one raw egg and when this is smooth add two teaspoons of tarragon vinegar.
This sauce is good for fish dishes and cold meats.