Thinly slice medium 3 leeks and wash them very thoroughly. Cut 150g of smoked bacon into short strips and place in a deep pan with 40g of butter. Place over a moderate heat and let the bacon cook for a few minutes until the fat turns golden. Turn down the heat and add the leeks, then cover with a lid. Leave to cook for 20 minutes or so, until the leeks are soft and sweet. Check occasionally, so they don’t colour. Have 750g of mussels ready, washed and checked, discarding any cracked or broken shells or any that don’t close instantly when tapped on the side of the sink. Put them in a large pot, pour in 100ml of white vermouth or white wine, then cover tightly with a lid and bring to the boil. Cook for a minute or two, then check their progress. If the mussels have opened, remove from the heat. If not, give them a minute or so longer. Remove the mussels from their shells, reserving the cooking liquor and discarding the shells. Sprinkle 2 tbsp of flour on the leeks and continue cooking for 2 minutes, then pour in the cooking liquor from the mussels, using a fine sieve to catch any shell or grit. Pour in 250ml of fish or vegetable stock, bring to a boil, then stir 250ml of single cream into the pot and bring back to the boil, seasoning it with salt (you may only need a little) and black pepper. Chop a generous handful of parsley and stir in. Add the mussels and ladle into bowls. Enough for 4 You can add water in place of the stock. There is already plenty of flavour from the mussel’s cooking liquor. I sometimes make this a little more substantial by toasting slices of baguette and putting them in the bowl before ladling over the soup. The Observer aims to publish recipes for seafood rated as sustainable by the Marine Conservation Society’s Good Fish Guide