Stir the cheese, onion, egg yolks, milk, seasoning and flour into the potato purée.
Beat the egg whites until stiff and fold into the potato mixture. Heat the oil in a wide frying pan and cook tablespoons of the mixture until barely golden. Eat at once.
Castilian mercenaries fighting in Germany during the Thirty Years’ War began carrying potatoes as provisions for horses and, in dire circumstances, for themselves.
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