Wash the chicken thoroughly and pat dry. Wash herbs, pat dry and chop except for something to garnish. Wash lemon. Grate the zest from 1/2 lemon. Halve the lemon and squeeze the grated half.
Cut the remaining lemon half into slices. Rub chicken inside and out with salt and pepper and put 2 lemon slices and half of the herbs into the chicken. Tie legs and wings together with kitchen string.
Put chicken in a roaster. Place 1 tablespoon of fat in flakes on top. Peel onions. Clean and wash 1-2 carrots. Chop both roughly and distribute around the chicken. Roast in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 1 1/2 hours.
Little by little pour on hot chicken soup and 2-3 tablespoons of lemon juice. Add remaining lemon slices 1/2 hour before the end of cooking time. Mix remaining herbs and grated lemon peel and spread on the chicken 10 minutes before the end of cooking time.
For the vegetables, clean and wash kohlrabi and the rest of the carrots. Cut the kohlrabi into sticks, cut the carrots in half lengthwise and cut into diagonal pieces. Cook vegetables in little boiling salted water for 15-20 minutes.
Pour the vegetables onto a sieve, drain well. Melt the remaining fat in a pot, toss the vegetables in it. Keep warm. Take the chicken out. Pour frying stock through a sieve. Add lemon jelly and cream, bring to the boil and pour in the sauce thickener while stirring.
Bring to the boil again briefly and season to taste with salt, pepper and possibly a little sugar. Arrange the chicken on the vegetables and sprinkle with remaining chervil and garnish as desired.