Scrub the mussels thoroughly and remove the beards. Discard any with open shells that do not close when sharply tapped.
Melt the butter in a cooking pot large enough to hold the mussels. Add the onion, garlic and bacon and cook gently for 3-4 minutes to soften the onion. Add the cider and simmer for a minute or so. Add the mussels, cover with a tight-fitting lid and cook over a high hea