Melt the butter in a large pan. Stir in the flour and cook for 1 minute. Remove from the heat and gradually blend in the milk. Heat, stirring, until thickened. Beat in the egg yolks and the cheeses. Season with the mustard, and salt and pepper to taste. Whisk the egg whites until thick. Using a metal spoon, fold a quarter into the cheese mixture, then fold in the remainder. Turn into an oiled
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