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Cream-of-Potato Soup
1 cup mashed pota- 1 tablespoonful buttoes 3 cups milk ter 1 tablespoonful flour Salt and white pepper 1 slice of onion Heat the milk and onion together, but do not boil; pour over the cold mashed potato and press II4 through a purée strainer. Melt butter, stir in flour and let bubble, but not brown; cool slightly and add the milk mixture slowly, stirring constantly until thickened. Add the seasoning and serve very hot, sprinkling a few nicely browned croutons or a little grated cheese over each plate of soup.
A thin béchamel built on leftover mashed potato, and a good use for the bowl in the refrigerator. The ingredients are one cup mashed potato, three cups milk, one tablespoon butter, one tablespoon flour, a slice of onion, salt and white pepper.
- The hingeEverything depends on pressing the potato-and-milk mixture through a sieve. Mashed potato stirred straight into hot liquid stays lumpy and can turn gluey; sieving breaks it down without overworking the starch. A fine mesh strainer and the back of a ladle do the work of a purée strainer.
- Do not boil the milkThe instruction is deliberate. Milk heated with onion picks up flavour without boiling; brought to a rolling boil it skins over and catches on the pan bottom. Heat until it steams and small bubbles form at the edge, then hold it off the heat to infuse.
- RouxButter and flour cooked until they bubble but do not colour is a white roux, roughly two minutes. Cooling it slightly before adding warm liquid is the old insurance against lumps; adding the milk in a thin stream while whisking does the same thing.
- Reheating starchOnce thickened, keep the soup below a simmer. Boiling a potato-and-flour soup hard will thin it again as the starch granules break down, and scorching is easy in a thin pan.
- ConsistencyThree cups of milk to one cup of potato gives a pourable soup. Leftover mash that already contains butter and cream will thicken more than plain potato, so hold back some of the milk and adjust at the end.
- White pepperSpecified for appearance rather than heat, so black flecks do not show in a pale soup. Black pepper works if the look does not matter.
- YieldAbout one quart, enough for three or four small first-course servings. Croutons or grated cheese are worth the trouble; the soup is plain without them.
Left-over cookery of the early twentieth century was aimed at households that bought food in quantity and treated waste as a fault as much as an expense. Mashed potato was an obvious surplus: cheap, made in bulk, and dull on its second appearance in its original form. Turning it into a cream soup by way of a white roux and infused milk is French technique applied to thrift, the sieve doing what a blender would now do in seconds.