Persillade Sauce

A cooked-yolk emulsion in the family of gribiche and rémoulade, thickened by egg yolk alone with no raw egg involved. It comes together like mayonnaise and fails the same way, so the oil goes in slowly.

• Gills and volumesTwo gills of oil is one cup. Four tablespoonfuls of vinegar is a quarter cup; the juice of four lemons is closer to a half cup, so if you take the lemon option expect a noticeably looser, sharper sauce and add it to taste rather than all at once.
• MignonetteMignonette (or mignonnette) pepper is coarsely cracked white peppercorn; some definitions also take in a little coriander seed. Ranhofer lists it alongside pepper, which suggests he wanted both the ground heat and the visible crushed grains.
• Pounded yolksRather than a mortar, press the hard-boiled yolks through a fine sieve. Any lump of yolk left whole will refuse to emulsify and show up as a grain in the finished sauce.
• The hinge: oil by degrees"Dilute this gradually" is the whole technique. Work the mustard and sieved yolks to a smooth paste first, then add oil a few drops at a time until it takes, alternating with small additions of the acid. Cooked yolks emulsify less forgivingly than raw, and a rushed pour will break it.
• Oil choiceNineteenth-century 'oil' in a book of this kind almost certainly means olive oil. A robust modern extra-virgin can dominate the herbs; a mild olive oil, or half olive and half neutral, probably sits closer to the balance intended.
• HerbsParsley, chervil and tarragon should be chopped at the last moment and folded in, not pounded with the yolks, or the sauce turns muddy green. If chervil is unavailable, increase the parsley slightly rather than substituting more tarragon, which is assertive.
• Yield and serviceAbout a cup and a half, enough for six to eight. It is served separately, at room temperature, with cold boiled beef, poached fish, calf's head or plain cold poultry. Refrigerate no more than two days; it stiffens when cold and needs a stir and a few minutes out before serving.