Green Spanish Sauce

A bread-thickened green sauce in the older French manner, closer to a sieved salsa verde than to a true mayonnaise. It has no egg in it at all, which changes how it must be handled.

• The hinge: the bread is the emulsifierWith no yolk, the squeezed bread-crumb paste is what holds the oil. Add the oil in a very thin stream while beating hard; if you rush it the sauce breaks and looks curdled and oily rather than smooth and opaque.
• Weights of herbsAn ounce each of chervil and parsley, most likely leaves rather than stems — a generous handful of each. Chervil is scarce; parsley with a little tarragon is a common modern substitute, though the anise note will be stronger.
• Bread-crumbsPresumably the soft crumb of a stale white loaf rather than dry packaged crumbs, since the crumb is to be soaked and squeezed. Four ounces is roughly three cups loosely torn. Soak in cold water and wring out hard in a towel, or the sauce will be watery.
• Sweet oilNineteenth-century American term for a mild, non-rancid olive oil. Quantity is unstated: expect ¾ to 1 cup, added until the sauce takes on gloss and body.
• Spinach greenVert d'épinards, made elsewhere in the book by pounding raw spinach, pressing out the juice, and heating it gently so the green coagulates for straining. A teaspoon or two of spinach purée does the same job here; it is for color only, so add it at the end.
• Anchovies and capersSalt-packed anchovy fillets rinsed of their salt are closest to what was meant. Between the fillets and two ounces of capers the sauce is already salty — taste before adding any.
• Pounding and sievingA mortar and a drum sieve were the tools. A food processor does the pounding, but still press the paste through a medium sieve afterward: the smooth texture is the point of the recipe.
• Yield and serviceServe cold with poached or cold fish, cold boiled beef, or hard-boiled eggs. It holds two or three days refrigerated; the green dulls after the first day.