How to cook
The bought mussels need to have closed shells. If there's open or broken ones, set them aside.
Put the mussels in cold water. Brush them with a brush to remove the polyps.
Put them in a suitable pot on the stove, add the beer and butter. Season with salt.
They don't need to boil long; 4-5 min. is enough for them to open up.
Serve with a suitable salad and drink.