Preheat the oven to 220 C / Gas 7. Butter 6 ramekins.
Melt the butter in a saucepan, then add the flour. Mix well with a wooden spoon. When the mixture has cooked for 5 minutes, add the milk and mix well to thicken. Bring to the boil, stirring constantly for 5 minutes.
Beat egg whites till stiff; set aside.
Combine the egg yolks, Gruyere, salt and pepper. Slowly stir in the still warm milk mixture. Gently fold in the egg whites.
Pour into the ramekins filling no more than 3/4 full. Bake 15 minutes without opening the oven. Serve immediately.