Bring 1/2 cup of water to a boil in a large saucepan and add the mussels. Cover and simmer for 5 minutes. Strain the mussels through a cheesecloth-lined sieve, reserving the cooking juices. Remove the mussels from their shells and discard the shells. Scald the milk in a medium saucepan. Melt the butter in a saucepan and cook the leek and potatoes until softened, about 10 minutes, stirring occasionally. Pour the milk into the vegetables. Add enough water to the reserved mussel cooking liquid to make 2 cups and add to the milk, along with half of the bread crumbs. Simmer for 30 minutes. Season with salt and pepper. Stir in the mussels and heat through. Stir the chervil into the soup just before serving.
