Wash and scrub the mussels in several changes of cold water. Discard any with broken shells or ones that are not tightly closed. Pull away the beard from each one.
Rub the inside of a large saucepan with a buttered paper and sprinkle with the chopped onion. Put the clean mussels in the pan and add the water and wine. Cover the pan with a lid and place over high heat. Shake the pan occas