Melt the fat in a pot. Stir in flour, sweat briefly. Add milk and stock little by little while stirring. Bring to the boil, simmer for 6-8 minutes, remove from heat. Season to taste with salt, pepper and nutmeg.
Roast pumpkin seeds in a pan without fat. Cut off cabbage, wash and drain. Peel, wash and cut kohlrabi into thin slices. Peel carrots and cut into thin slices.
Spread about 2/3 of the sauce in an ovenproof dish (approx. 17 x 25 cm). Place kohlrabi, carrots and ham alternately close together on edge.
Wash the chervil, shake dry and pluck the leaves from the stems. Finely puree the chervil, pumpkin seeds, kohlrabigrün, lemon juice, 1 teaspoon salt and olive oil.
Drizzle some pesto and the rest of the sauce over the vegetables. Cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 40 minutes. Take out, arrange and add the rest of the pesto.