Egg Sauce

A short finishing sauce of the old English kind, meant to be spooned over boiled fish or shellfish. The recipe assumes you already know how to make white sauce, and it assumes the raw egg will thicken in the heat of it.

• The white sauceThe recipe assumes a white sauce you already have on hand; a plain flour-and-milk white sauce of the usual sort (roughly 2 tbsp butter, 2 tbsp flour, 1 cup milk, cooked until it no longer tastes of raw flour) suits the two eggs here, though the original specifies no quantity.
• The hinge: temper the raw eggBeat the raw egg in a bowl, whisk in a few spoonfuls of the hot sauce to warm it, then return the mixture to the pan off the direct heat. Poured straight into a simmering sauce it curdles into threads.
• Then bring it upOnce the egg is in, hold the sauce over low heat, stirring, until it thickens slightly and reaches 160F. That both sets the egg safely and gives the sauce its gloss. Do not let it boil.
• Hard-boiled eggChop it finely and add it last so it stays in visible flecks rather than dissolving. A yolk-heavy chop gives a richer colour; some cooks sieve the yolk and chop only the white.
• CapersIf using, drain and rinse brined capers and add a scant tablespoon at the end. They are salty, so season the sauce after they go in, not before.
• Yield and servingIf you start from about a cup of white sauce, you will get roughly 1 1/4 cups. Best made just before serving; it thickens and dulls on standing and cannot be boiled back.