Melt the butter in a saucepan, medium flame, and pour the flour on it while stirring with a wooden spoon until well blended.
On a more gentle flame, start pouring a bit of milk and gently stir until it is incorporated ; keep doing this way with all milk.
Never stop stirring. Cook about 10 minutes until the white sauce is ready (it must get thick).
Add enough nutmeg, salt & pepper, and remove from fire.
Add the grated cheese, then the yolks ; turn vigourously to obtain a homogeneous mixture.
Gently incorporate the egg whites with a rubber spatula.
Pour the mixture equally into the 4 soufflé dishes ; fill them about 3/4.
Bake in oven for about 25 minutes ; the soufflés must rise and get golden brown.
Do not open oven door for they would deflate.
You need to increase baking time if you use a large single dish, and vice versa. It would take around 40 minutes for 1 big dish...