In a large pan, melt the butter and add the leek, fennel and mushrooms (if using) the shallots, curry powder, saffron and thyme. Cover the pan and gently sweat the vegetables for about 5 minutes, or until soft but not coloured.
Add the wine and bring to the boil, then add the mussels. Cover and cook, shaking occasionally until all the mussels are open. Discard any which remain closed.
Strain and return the liquor to the pan. Cook uncovered for 5 minutes to reduce before adding the cream and parsley.
Bring back to the boil, return the mussels to the pan, allow to heat for a minute or so and serve immediately with crusty bread.