Make a roux by melting the butter in a saucepan and then stirring in the flour till it's hot and bubbly. Stir in the mustard. Scald the milk in another saucepan.
Preheat oven to 190 degrees C.
Gently stir the milk into the roux using a wooden spoon over a low heat and keep stirring till all the lumps have gone and the sauce starts to thicken- about 3 minutes. Once the sauce is nice and thick take it off the heat and stir in the grated cheese.
Separate the egg yolks and whites. Lightly beat the yolks and whip the whites till stiff but don’t over beat them once they're there.
Mix the yolks into the still warm/hot cheese sauce until well mixed. Add a large spoonful of the whites and mix in well. Then fold the rest of the whites gently in to the roux.
Pour immediately into a well greased straight sided 20cm cake tin or pyrex dish and cook for 30 mins. Serve immediately!