Melt the butter in a large saucepan and stir in the flour. Cook for 1 to 2 minutes over a low heat. Gradually add the milk, stirring well until the sauce is smooth and thick.
Cook gently for 10 mins stirring constantly, then remove from the heat and stir in the cream. Then add the egg yolks a little at a time, add the cheese, salt and pepper plus the nutmeg to taste. Beat well until the cheese melts and is really smooth.
Whisk the egg whites until stiff, fold them gently into the cheese sauce mixture.
Transfer the mixture to a well buttered 1.2 litre soufflé dish. Bake in a preheated oven at 350ºF for 15 mins. Increase the temperature to 400ºF and cook for another 15 mins until soufflé is golden and well risen.