Peel and halve the kohlrabis and cut out the inside, leaving a thin edge. Pre-cook the kohlrabi halves in boiling salted water for about 8 minutes and remove. Wash the herbs and chop except for some chervil for garnishing.
Cut the ham into small cubes. Separate eggs. Mix quark, milk, egg yolk and semolina. Fold in 2/3 of the herbs and ham. Season with salt and pepper. Beat the egg whites until stiff and fold into the quark.
Fill the quark filling into the kohlrabi halves. Put the kohlrabi on the fat pan of the oven. Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for 25-30 minutes. Peel and finely dice the onion.
Heat the oil in a pot and sauté the onion. Add kohlrabi scraps and stock, bring to the boil and simmer for 5 minutes. Puree the kohlrabi. Mix sour cream and starch and thicken the sauce with it.
Season to taste with salt and pepper. Stir in remaining herbs. Arrange filled kohlrabi with the sauce on plates and serve garnished with chervil.