Beat the egg yolks until pale in colour.
Mix in the cheeses, mustard and cayenne, nutmeg and salt to taste.
Whip the cream until it just holds its shape.
Fold into the cheese mixture with the prawns.
Whisk the egg whites until stiff, then fold into the mixture.
Spoon into 4 ramekin dishes and chill until required.
Serve garnished with prawns if using, and lemon slices.