Brush the mussels vigorously under cold running water, sorting out damaged and open ones. Remove the hard threads ("beard") with a small knife and scrape off lime deposits etc. Peel and finely dice the onions and garlic. Clean, wash and cut the leek into thin rings. Wash the parsley, pluck off the leaves and chop them coarsely.
Heat butter in a large pot. Sauté onions, garlic and leek in it until transparent. Stir in parsley. Deglaze with beer and bring to the boil. Season with salt and pepper.
Add mussels and simmer covered for about 10 minutes until they have opened. Remove closed mussels. Serve mussels. Bread tastes good with them.