Heat half the butter over a gentle heat in a heavy saucepan and sweat the vegetables, covered, for about 5 minutes, shaking the pan occasionally.
Meanwhile open the mussels in the cider, discarding any that remain closed. Remove the mussels from their shells, resrving the liquor in the pan. Discard the shells.
Carefully strain the liquor into the pan with the vegetables and add 1 3/4 pints water.Add the whole chilli and simmer for about 15 minutes until the vegetables are tender. Season .
Remove the chilli and liquidise the soup, then sieve back into the pan.
Just before serving bring the creme fraiche to the boil, stir into the soup and heat through to simmering point. Check the seasoning and add the mussels.
Serve with slices of french bread rubbed with garlic or toasted.