Put the mussels in a casserole with the wine and place over a high heat. Bring to the boil and cook until the mussels have opened, stirring frequently to ensure they cook evenly. It will only take a few minutes.
Strain the cooking liquid into a jug through a fine sieve. Discard any mussels that have not opened. Return the opened mussels in their shells to the casserole. Cover and set aside.
Melt the butter in a saucepan, add the onion and carrot and cook over a low heat for 2 minutes until softened but not browned.
Add the sultanas, curry powder and tomato puree and cook, stirring, for one minute. Gradually stir in the strained mussel liquid.
Peel the apple, cut into wedges and add to the sauce. Let the sauce simmer gently for 5 minutes then add the cream.
Pour over the mussels and heat through thoroughly. Serve at once with some frites or crusty bread to mop up the juices.