1. Clean the fish and poach it with water, salt, one cleaned and roughly chopped onion and four carrots, washed parsley, mace, thyme and peppercorns.
  2. Let it poach slowly for 30 minutes.
  3. Clean the mussels one by one, remove the mud mussels, the broken mussels and the mussels that are open and do not close when touched.
  4. Wash the mussels several times in cold water.
  5. Sieve the fish stock, bring it back to the boil and cook the mussels in it for 8 minutes.
  6. Drain the mussels well and collect the cooking juices.
  7. Clean and chop the rest of the carrots and onions, celeriac and potatoes.
  8. Wash the celery and finely chop it.
  9. Heat the butter and slowly fry the vegetables.
  10. Add the stock and the mussels.
  11. Bring the soup to the boil and season to taste.
  12. If necessary, thicken the soup with some mixed flour.
  13. Stir the cream through the soup just before serving.
  14. Serve the soup with white bread and butter.