Wash the potatoes thoroughly, bring to the boil in water and cook for 20 minutes over medium heat. Then drain, rinse and peel. In the meantime clean, wash, halve and slice the kohlrabi.
Clean, wash and slice the carrots. Add kohlrabi to the boiling broth and cook for 10 minutes. After half the cooking time add the carrots. Finely dice the ham. Pour the vegetables into a sieve and drain.
Pick up the broth. Peel and finely chop the onion. Heat the fat in a pot and fry the onions until transparent. Sprinkle with flour and sauté. While stirring, gradually deglaze with the collected stock and cream, bring to the boil.
Season to taste with salt, pepper and sugar. Cut the potatoes in half. Put potatoes, vegetables and ham in a greased, ovenproof dish and pour the sauce evenly over it. Sprinkle with cheese and bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 20 minutes.
Remove from the oven and serve garnished with chervil as desired.