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Duchess Soup
Slice 2 large onions and fry them ten minutes in 2 tablespoonfuls of butter. Add gradually 2 tablespoonfuls of flour and stir two or three minutes, or until smooth; then pour in slowly a quart of boiling milk, season with salt and pepper, and let it simmer fifteen minutes, stirring frequently. Strain and return it to the fire, and add 2 tablespoonfuls of grated Parmesan cheese. Beat 3 eggs very light, pour them through a strainer into the soup; stir briskly for a few minutes, but do not let it boil. Serve with toasted croutons or fried sippets.
A simple onion velouté finished with an egg liaison. Everything hinges on the temperature at the end: once the beaten eggs go in, the soup must stay below a simmer or it will curdle into strands.
- The hingeHold the pan below a simmer after the eggs are added -- roughly 170-180F -- stirring constantly, until the soup coats the spoon. If it boils the eggs scramble into strands, and whisking rarely brings it back. Tempering helps: stir a ladle of hot soup into the beaten eggs first, then return the mixture to the pan off the heat.
- Fry the onions gentlyTen minutes in butter is meant to soften and sweeten, not colour. Keep the heat moderate and cover the pan; browning turns the finished soup grey-beige rather than ivory.
- QuantitiesA quart of milk is 4 cups. Two tablespoons each of butter and flour make a thin roux, so the body of this soup comes mostly from the eggs and cheese, not the flour.
- Boiling milkThe recipe calls for the milk boiling, as written; add it hot and in a slow stream while whisking so it incorporates smoothly.
- StrainingThe onions are strained out, so slice rather than chop and press the solids firmly in the sieve to get the flavour through. If you would rather keep the body, a stick blender before straining gives a fuller soup.
- ParmesanGrate it fresh and add it off the direct heat; pre-grated cheese carries anti-caking agents that can make the soup grainy.
- SippetsSippets are small triangles or fingers of bread fried in butter, the older sibling of the crouton. Either works; fry them just before serving so they stay crisp.
- YieldAbout four servings as a first course. Serve immediately, as the egg-thickened soup will not reheat without separating.
Nineteenth-century soup nomenclature borrowed freely from the peerage, and "duchess" was applied to any number of pale, delicate, egg-and-cream-enriched dishes, much as "duchesse" potatoes are potatoes piped and enriched with egg yolk. What is being made here is a liaison -- beaten egg stirred into hot liquid, which sets just enough to give a silky body -- one of the older thickeners in European cookery, alongside bread and ground almonds, and long predating the flour-based roux.