the idea of a load of mussels swimming in a bowl of porridge!
Most supermarket bought mussels have already been pretty much cleaned so can be cooked as soon as you get home, you still get some grit in them though, especially if they've got those barnacle type things that look like mini volcanoes on the shells. They, and the string (called the beard) the mussels use to attach themselves to things, can be cleaned off with a bit of persistence and a sharp knife, best done in water to keep washing the bits off. But be careful how you handle them, if they get too shaken up (and the strings pulled too violently) the beast inside the shells will die and they go off very quickly. Throw away any with broken shells. We put the cleaned mussels in a bowl of salt water (remember; they live in the sea) with a small amount of porridge oats in the water. Ever couple of hours give the bowl a stir (they live at the edges of the tidelines and are used to being washed through with water, it's how they feed). After a while they start to open up, although they'll close if stirred too hard. After six hours or so we empty the bowl and thoroughly wash them again to get rid of any further grit and muck, throw away any that don't close completely on tapping on the work surface top. You can go on to the cooking stage from here if you're ready to eat otherwise leave them in an empty bowl for up to six hours (in the fridge if it's warm in the house). If you want to keep them longer you should imitate the tides and alternately cover them with salt water for six hours or so before draining them for six hours etc.
Whatever, give them a further hour in salt water before cooking. Always discard any that don't close on tapping on a hard surface. We generally reckon on losing anything up to a third of them over 24 hours. Drain the water out of the bowl.
Finely chop a shallot and soften it in butter in a deep saucepan (big enough to hold the mussels and it must have a lid). Add the mussels, put on the lid and shake. Add about a quarter of a bottle of dry white wine; Muscadet de Sevre et Maine is technically the correct one (although some swear by white Burgundy or a good chardonnay) but any dry white will do, (you can also use cider). Bring to the boil, uncovered, to boil off the alcohol then cover and steam for a few minutes, shaking occasionally, 'til the mussels have opened (only a few minutes), chuck in a good dollop of cream, bring back to boil, chuck in a handful of not too finely chopped parsley, ladle into bowls with the cooking liquor and serve with a chunk of crusty bread and a glass of the wine with an empty bowl on the side for the shells.
Posh people use one of the shells, still hinged, as pincers to pick out the mussels, I tend to use my fingers, but you can use a small fork and spoon if you like. DON'T EAT ANY THAT HAVEN'T OPENED.
It is a bit of a rigmarole, and I know lots of people who aren't nearly as careful who've suffered no ill effects, but shellfish poisoning can be very unpleasant.
Currently investigating the Hillmans of Sussex.