Olive Sauce for Roast Duck

A short sauce that depends on two things: getting the salt out of the olives and keeping the white sauce from breaking.

• Which olivesThis calls for salty brine-packed olives with the stone still in, since the paring runs "through to stone." Modern pitted olives can be substituted, but you lose the spiral of flesh that gives the sauce its look; if you use them, slice into thin rings instead.
• The paring cut"Pare round and round through to stone" means cutting a continuous ribbon off the olive the way you would peel an apple, so the flesh comes away in one curl. A small sharp paring knife and a stone-in olive make this easier than working a slippery pitted one.
• Soaking is not optionalTwenty minutes in hot water draws out some of the brine, which is why the recipe adds no salt of its own. Skip it and a cup of olives risks overwhelming a cup of white sauce. Taste one before you commit; olives vary widely in saltiness and some may need a change of water.
• The white sauceThe recipe assumes you already know how to make a cup of white sauce: butter and flour cooked together, milk whisked in, simmered until it coats a spoon. It does not specify a thickness, and period ratios ran from about 1 tablespoon each of butter and flour per cup of milk for a thin sauce up to 2 tablespoons each for a thicker one.
• Eight minutes, gentlyHold the pan at a bare simmer. A cup of milk-based sauce over direct high heat will scorch on the bottom in far less than eight minutes.
• YieldRoughly a cup and a half to two cups of sauce, enough for one roast duck. Serve it hot in a boat rather than poured over, so the skin stays crisp.
• Seasoning at the endThe recipe gives no seasoning at all. Taste before serving; black pepper, a spoonful of the duck pan juices or a squeeze of lemon will lift it if it tastes flat.