(serves about 15)
5kg mussels debearded and scraped clean
110g unsalted butter
1kg leeks washed trimmed and sliced
300g main-crop potatoes peeled and sliced
150ml dry white wine
450ml strained mussel juice
150ml double cream
Salt and pepper
45 fresh rock oysters opened and left in their juice
Cream and chives for garnish
Cook mussels in a large covered saucepan (in batches) until just opened then remove them from their shells and reserve the liquor. Strain it through muslin.
Melt butter in another large covered saucepan and sweat leeks and potatoes for 5 minutes. Add stock wine mussel juice and cream. Season lightly with salt and pepper but check saltiness of mussel juice before adding more salt. Simmer until vegetables are just cooked (about 15 minutes) and blend in a liquidiser.
For each person reheat a large ladle (about 200ml) of soup together with six mussels and three oysters with their juice. Bring to the boil and serve garnished with cream and chives.