Cut the cauliflower into florets. Dice potatoes. Cook both in 1/4 litre boiling salted water for about 15 minutes. Remove some cauliflower florets after about 8 minutes and put them aside. Pluck the chervil leaves.
Dice fish finely. Season with salt and lemon juice. Add chervil, except for a few leaves for sprinkling, and puree. Cut off 4 dumplings with 2 teaspoons and let them simmer for 3-5 minutes in lightly boiling salt water.
Mash cauliflower and potatoes in the cooking water. Stir the cream into the soup and season to taste with salt, pepper and lemon juice. Heat the remaining cauliflower florets and dumplings in the soup. Arrange and sprinkle with remaining chervil