It is a thick yellow sauce that looks like tartar sauce or mayonaise, but hot and contains minced shalots, mushrooms, truffles and parsley.

Put into a small bright saucepan 4 tablespoons vinegar and 1 of minced young onions, and boil; add 2 ounces best butter (large egg size) and then 4 yolks and stir over the fire until it begins to thicken; add 1 tablespoonful each of minced mushrooms and truffles, little salt, cayenne and finely minced parsley. It is to be cooked enough to set the egg yolks to a buttery thickness, but not enough to cause them to break into curds.

There was a king called Henry the Bearnaise. The word refers either to him or his country.