Heat the oil and fry the mince until crumbly. Steam the finely chopped onion. Salt, pepper. Let everything cool down a bit.
For the bechamel sauce, heat the butter. As soon as it froths up, add flour and stir quickly with a whisk. Slowly pour in the hot milk, stirring constantly, bring to the boil and allow to swell for 15 minutes. Season to taste with salt, pepper and nutmeg. Stir the yoghurt into the slightly cooled sauce.
Peel the kohlrabi, carrot and potatoes and cut into fine slices with a cucumber slicer. Grease a wide baking dish, layer the vegetables like roof tiles and sprinkle the minced meat crumbs in between. Salt and pepper. Finally, spread the bechamel sauce over the vegetables so that everything is covered. Put flakes of butter on top.