The real French purée of potatoes is something so vastly different from our “mashed potatoes,” and so typically French, that it cannot be omitted from a book on French cookery.
Peel and quarter the potatoes and put them in a saucepan with just enough boiling salted water to cover. Cover the saucepan, bring to the boil and boil for 20 minutes. Drain thoroughly, and rub, as rapidly as possible, through a wire sieve, into a basin. With a wooden spoon mix well, season with salt and pepper, and start adding the butter, divided in small pieces stirring vigorously and quickly. When the butter has been completely absorbed, start adding the milk and keep on stirring or “beating” the purée so that it is very light. When done it should be almost frothy. Put the purée back in a saucepan and warm up, still stirring, but do not let it boil.
A well-made purée should be almost “runny”—only just sufficiently thick to be eaten with a fork.