For kohlrabi au gratin, preheat the oven to 190-200° C (top / bottom heat).
Mix 125 ml of milk with water and pour into a saucepan with the kohlrabi and kohlrabi flakes, season with salt and simmer gently over moderate heat for 10 minutes until the vegetables are cooked but still firm to the bite. Drain, reserving the liquid.
Heat 150 g butter in a saucepan, sprinkle in flour and sweat until golden. Add 125 ml milk and cooking liquid. Cook for a few minutes, stirring constantly. Season with salt, nutmeg and pepper, remove from heat and add the vegetables.
Grease four ramekins with a little butter.
Mix egg yolk and cheese, stir into the vegetables.
Fold the egg whites into the mixture, beating them stiff with salt.
Pour the mixture into the ramekins and bake for 15-20 minutes.
Serve kohlrabi au gratin as an appetizer with garnish.