Wash the potatoes and cook for about 20 minutes. Quench, peel and let cool. Cut into thin slices
Peel, wash and slice the kohlrabi. Finely chop some kohlrabigrün. Boil up about 1/2 l water with broth. Cook the kohlrabi for 10 minutes. Drain, catch the broth
Wash the fillets, dab dry. Fry in hot oil for about 7 minutes on each side, season. Cut the fillets open. Boil off the meat with the broth
Melt the fat. Sweat flour in it. Stir in stock, bring to the boil. Grate cheese. Stir sour cream, half cheese and kohlrabigrün into the sauce. Season to taste with salt, pepper and nutmeg
Fill the kohlrabi, potatoes, fillets and sauce into an ovenproof dish. Sprinkle with the rest of the cheese. Bake in a hot oven (electric cooker: 200 °C / convection oven: 175 °C / gas: level 3) for about 30 minutes. Garnish with chervil