Grease and sprinkle some breadcrumbs in porcelain custard cups. Heat 2 tbsp (30 g) of butter in a pan on the stove and fry the flour in it, stirring constantly for 2 minutes.
Remove the pan from the heat and while stirring continuously, pour in half of the milk. Stir until homogeneous and gradually top up the remaining milk.
Return to the heat and continue to stir, wait for it to boil and thicken. Remove from the heat and add the grated cheese and mustard. Then add the egg yolks, stir and season with salt and pepper. Beat the egg whites separately to hard snow with a pinch of salt and gently blend them into the egg-cheese mixture.
Distribute into the cups and put to bake for about 15 minutes in a preheated 392°F (200 °C) oven. Once they swell and get a nice tan, remove and serve immediately.