Give the mussels a quick rinse under cold running water. You’ll need to pull away the grey beards and knock off any barnacles using the back of a knife . Discard any mussel shells that fail to close after a tap.
Place a large heavy-based pan with a tight fitting lid over a high heat. Add 150ml of water and the bay leaves and parsley stalks.
When the water is boiling rapidly add the mussels, give the pan a shake and pop the lid on. Cook for 2-3 minutes, giving the pan a further shake, until the mussel shells are all open.
Remove the pan from the heat and immediately tip the mussels out into a large colander set over a bowl to catch the cooking liquid. Don’t forget the bowl or pour the mussel liquor down the sink. It’s an important part of the soup.
Rinse the pan out and return it to a medium heat. Drop in the butter and when it’s bubbling add the pancetta or bacon and fry it slowly until it’s just beginning to crisp around its edges.
Now add the coriander seeds, garlic and leek along with some salt and pepper, and carry on cooking, stirring occasionally, until the leek is soft and silky, about 8 minutes.
Add the mussel cooking liquor (except the last few drops which can sometimes be gritty) and the stock. Bring the soup to a simmer and cook uncovered for 5 minutes.
Pick the meat out of the mussel shells, keeping a few intact to garnish, and set to one side. Add the cream to the soup and bring back to a simmer. Now add the mussels and chopped parsley, and plenty of black pepper and salt to taste.
Arrange the reserved shells in the base of four warm bowls. Ladle over the hot soup and serve at once.