Fry the onions in butter. Fry the kohlrabi and potatoes. Pour 3/4 l water, bring to the boil. Season with broth, salt and pepper. Cover and simmer for about 20 minutes.
Add the chervil to the soup, keep some for decoration. Briefly puree the soup. Slightly whip and stir in the cream, season to taste and briefly foam up again with the hand blender. Arrange immediately and serve sprinkled with chervil and radishes.