Peel the potatoes and kohlrabi, cut into thin slices and layer them alternately in a baking dish. Season each layer with salt and pepper.
Melt the butter, add the flour and sweat. Add the milk and bring to the boil. Then add the cream, season with salt, vegetable stock and nutmeg and pour the sauce over the layered vegetables.
Bake at 175 ° C for about 30 minutes. Then sprinkle the grated cheese on top and bake in the oven for another 20 minutes.