Open the bag of mussels and tip them all into a large bowl.
If you wish, sprinkle some oats over the surface of the water and place the bowl of mussels in the fridge. Leave for 4 hours or overnight to purge. The mussels will absorb the oats and will release any sand or grit that may be inside.
Drain the water and then take each mussel. Hold in one hand and pull the fibrous beard from the shell. Scrape off any barnacles if you wish.
Tap any open shells and watch to make sure that they firmly close up. Discard any broken mussels and those that will not close tightly.
Place the whole tightly closed mussels into a large pan. Add a splash of water. Cover with a lid and then bring up to the boil. Cook for 5 minutes, stir the mussels and check to see if the shells have opened. If the shells have not opened up, cover with a lid and cook for a further 5 minutes.
Using a small fork or by pincing an empty shell together, pull out the cooked mussels. For the ultimate guide to preparing and cooking mussels, see our video.