Soubise Soup
Peel and slice thinly six white onions, put them in the soup kettle with one tablespoonful of butter and cook for ten minutes without browning. Add one quart of water, one pint of milk, one-half teaspoonful of salt, one-quarter of a teaspoonful of white pepper, a pinch of mace and onehalf of a teaspoonful of sugar. Cook slowly for one hour, then rub through a purée sieve. Return to the fire, add one teaspoonful of cornstarch dissolved in a little cold water and bring again to the boiling point. Simmer for two minutes, then draw to the side of the fire and stir in the beaten yolks of three eggs mixed with one cupful of cream. Take at once from the fire and serve with croutons.
A classic white onion purée finished with an egg-and-cream liaison. Two things decide whether it works: keeping all colour out of the onions, and keeping the finished soup off the boil.
- The hinge: no browningThe onions go into butter for the full ten minutes with no colour taken; that means gentle heat and stirring. Any colour at all and the soup turns beige and tastes of French onion soup rather than soubise, which should be pale ivory and sweet.
- The liaison will curdle if it boilsOnce the yolks and cream go in, the soup must not return to a simmer. Temper by whisking a ladleful of hot soup into the yolk-cream mixture first, then stir it back in off the heat. Reheating leftovers is best done gently and well below boiling.
- Sieve or blenderA purée sieve is what the recipe calls for; a blender or immersion blender does the work in seconds. Passing the blended soup through a fine strainer afterwards still pays off, since onion fibre is stubborn.
- One teaspoon of cornstarchThat is a very light hand for six cups of liquid, and deliberately so: it acts as a stabiliser rather than a thickener, since the egg yolks do most of the body-building. Slurry it in cold water before adding, as the recipe directs.
- White pepper and maceBoth are there to season without specking the soup. Mace is the outer coat of the nutmeg seed, milder and less resinous than nutmeg itself; a scrape of nutmeg substitutes, but use less.
- "Draw to the side of the fire"On a coal range this meant moving the pot to a cooler zone. On a modern hob, turn the heat off entirely and slide the pan to a cold burner.
- YieldRoughly six cups, serving four generously or six as a first course. Croutons matter more than usual here, because the soup is smooth and mild and wants something crisp.
Soubise is one of the standing French names for onion, applied to a white onion purée or a béchamel-based onion sauce, and it is generally traced to the household of the eighteenth-century Prince de Soubise, whose cook is traditionally credited with the preparation — a story repeated throughout nineteenth-century cookery writing without much documentation behind it. By the time American compilers like this one were at work, soubise had become shorthand in English-language kitchens for anything built on sieved onion, and this soup shows the era's fondness for the egg-yolk-and-cream liaison, a finish inherited from French practice that gave a thin purée the richness of a cream soup without the heavy flour roux. The insistence on white onions, white pepper and colourless butter belongs to the same period aesthetic that prized pale, refined soups at the head of a formal dinner.