Prepare the mussels: discard any with broken shells. Rinse them really well in a sieve under flowing cold water. Pull out any ‘beards’ or bits of seaweed. If any mussels are still open, give them a little tap with a spoon - if they still don’t close then throw them away.
Peel and chop the onion and garlic. Roughly chop the dill.
Put everything into a medium size casserole or heavy-bottomed pan with a lid, and put over a high heat.
Keep covered but stir well every minute or so for about 5 minutes, by which time they should all be open. Discard any that remain closed.
Pop in the middle of the table and dig in! Use crusty bread to dip into the delicious juices.