Peel and finely chop the onion and garlic. Steam them in hot fat until translucent. Add wine and 1/2 litre of water and bring to the boil. Wash the mussels thoroughly and add to the stock. Let them simmer for eight to ten minutes.
Remove the mussels and put them aside. Let the stock boil down by half in 30 minutes. Clean and wash the leek and fennel and cut into narrow strips. Cut the bacon into strips as well and leave to drain in a pan. Add leek and fennel, fry briefly and dust with flour. Stir well. Gradually add the stock and cream, stirring continuously, and bring to the boil. Peel and wash the potatoes and cut them into small cubes. Add them to the soup with the marjoram and cook on low heat for 30 minutes. In the meantime, remove the mussel meat from the shells and add to the soup.
Stir well. Gradually add the stock and cream, stirring continuously, and bring to the boil. Peel and wash the potatoes and cut them into small cubes. Add them to the soup with the marjoram and cook on low heat for 30 minutes. In the meantime, remove the mussel meat from the shells and add to the soup. Season to taste with salt and pepper. Sprinkle with parsley and chives just before serving