Prepare the stuffing. Soak the mussels in a bucket of water for 30 minutes. Scrape the shells thoroughly with a knife and wash clean under plenty of running water. Prise the shells open with a knife from the straight side to make butterflies. Cut off the beards and clean.
Put the water and salt in a pan. Rest a colander or steamer above the water.
Stuff the uncooked mussels with