Reduce one pint of veloute (No. 415), or espagnole (No. 414) with half a pint of chicken essence (No. 387). Infuse a pinch of tarragon leaves in a gill of chicken essence, and add this infusion to the veloute or espagnole; strain all through a tammy, and just when serving throw in a tablespoonful of tarragon leaves cut in lozenges, and blanched in boiling water in an untinned copper pan.
Ranhofer writes for a brigade that already has stockpots and mother sauces going, so most of the work here happens elsewhere in the book. Two things decide whether the sauce is good: the strength of the reduction and how late the tarragon goes in.
The Epicurean (1894) is Charles Ranhofer's record of thirty-odd years as chef at Delmonico's in New York, and it is organised the way a French kitchen was: a handful of mother sauces, then dozens of small variations built on them by reduction and a single dominant flavouring. Sauce a l'estragon is one of the plainest of those variations, and a comparable sauce appears in Escoffier a decade later. Tarragon reached French cooking as a herb strongly associated with poultry, eggs and vinegar preparations, and its name -- estragon, from a Latin form generally traced back through Arabic to a word for dragon -- is usually explained by the coiled, serpentine look of its roots, though that explanation is traditional rather than documented. The instruction to blanch the leaves in an unlined copper pan is a period fingerprint: the colour-fixing effect of copper on chlorophyll was well known to professional cooks long before anyone worried much about what else came off the metal.