Prepare the mussels.
Place the herbs, shallots, fish stock and white wine in a large saucepan and bring to the boil. When boiling, add the mussels, cover with a tightly-fitting lid and cook over a high heat until the mussels have opened. When ready, remove the mussels from the liquor and allow to cool. Discard any that are not open.
To make the sauce, pass the liquid through muslin or a fine strainer into a clean pan and then reduce until syrupy. Add the cream and the mustard and reduce until thick. Leave to cool.