Cooking the mussels
Place the washed mussels in a colander and give them a final rinse through in fresh water before cooking.
Melt the butter in a large pan with a close-fitting lid. Soften the onion and garlic in the hot butter until soft and translucent.
Stir in the parsley and add some freshly ground black pepper.
Pour in the wine and water and bring to the boil.
Add all of the mussels, lower the heat and cover with a lid and leave to simmer until the mussels have steamed opened. This will take no more than five minutes. Avoid opening the lid, but give the pan a good shake around once or twice during the steaming process.
(If you want to serve mussels traditionally like 'Moules a la mariniere' cook them to this stage and serve them in warm bowls with the cooking liquor poured over them. Sprinkle with extra chopped parsley and chives when serving. This quantity of mussels will be enough for four servings in this way.)
Stand the colander in a mixing bowl. Remove the mussels, using a slotted spoon, placing into the colander. Leave to drain and cool.
Ladle the cooking liquor from the pan into a large measuring jug through a fine sieve and reserve. Add any residual liquid from the bowl underneath the colander. Rinse out saucepan.
Making the brose
Finely chop the onions, celery, garlic and leek, including the green tops.
Dice the peeled potatoes into small cubes.
Melt the butter until hot and foamy. Soften the onion, celery and leek in the hot butter until soft and translucent.
Add the potatoes and stir together with the vegetables. Allow to cook gently for five minutes, stirring from time to time.
Pour in the strained mussel liquor and bring to the boil. Turn down to simmer for at least 30 minutes. The original quantity of white wine and water will have more than doubled, to give you approximately 1200ml, plenty to cover the vegetables. As the mussels steam open, the juices mix with the white wine and water, adding a natural saltiness.
Liquidise at this stage if preferred, or leave the soup to serve as a chunky version.
Add the oatmeal, stir well and simmer for a further five minutes.
Meanwhile, remove the cooled mussels from their shells and reserve in a bowl. Retain a few whole for garnish. Amongst the mussels there will be some residual chopped onion, garlic and herbs from when they were cooked. This is fine and all can be added to the soup.
Stir in the shelled mussels, together with the freshly chopped parsley and chives and some ground black pepper.
Before serving, stir in the carton of single cream and fill the empty carton with cold water and pour that in too.
Reheat and check thickness and seasoning. Salt should not be necessary. Add a little more hot water if too thick.
Serve hot in warmed bowls with whole mussels placed on top for garnish and a pinch of chopped chives on top. With some warm crusty bread, oatcakes or scones, this is a meal-in-one.