Melt the butter in a large saucepan and cook the onion and garlic for 2–3 minutes. Meanwhile, discard any mussels that are open and do not close when tapped.
Add the cider and mussels to the pan and cover with a lid. Cook over a high heat for 3–4 minutes, shaking the pan from time to time, until all the mussels have opened.
Discard any mussels which have not opened, then stir in the crème fraîche and herbs and heat through. Serve immediately with crusty bread.