Peel and chop the onion. Peel and wash the potatoes and kohlrabi and cut into small pieces. Melt the fat and fry the onion in it. Add the kohlrabi and potatoes, fry for 5 minutes at low heat. Season with salt and pepper. Deglaze with 800 ml water, bring to the boil. Cover and cook for about 20 minutes.
In the meantime clean and wash the radishes and cut them into very fine strips. Chervil roughly chop, whip cream a little bit. Add the chervil to the soup and puree it with the chopping stick. Stir in cream and season with salt and pepper. Arrange portions sprinkled with radishes. Garnish with chervil