Melt the butter in a saucepan large enough to hold the mussels. Add the leeks, fennel and garlic and fry gently for 5 minutes.
Add the stock and bring to the boil, then cover and cook gently for about 10 minutes until the vegetables are softened. Stir in the Pernod or other liqueur and bring back to the boil.
Add the mussels to the pan, cover tightly again and cook over a medium heat for 4–5 minutes until the shells open, shaking the pan occasionally. Discard any unopened mussels.
Carefully pour the cooking liquid into a small saucepan and set the covered pan of mussels aside. Stir the cream and chopped parsley into the cooking liquid and heat gently, without boiling. Season to taste with freshly ground black pepper. Tip the mussels and vegetables into a large, warmed dish or tureen and pour over the sauce. Serve at once.