Put in a spider a generous tablespoonful of butter and a cup of milk,
when hot add some cold potatoes cut in dice, season with pepper, salt, a
few drops of onion juice. Let them get thoroughly hot, then add the
beaten yolks of two eggs, stir constantly until thick. Great care must
be taken not to let it cook too long, or the sauce will curdle. Pour
into a vegetable dish, sprinkle a little finely minced parsley over the
top and serve.