Give the mussels a quick scrub; pull out the little beards and discard.
Peel the onion and slice into fine rings. Heat the butter and oil in a large lidded pan, and cook the onion gently for 5 minutes.
Add the white wine, chicken stock, celery and garlic cloves, and bring to the boil. Add the mussels, cover with the lid and cook for 1 minute, then shake the pan, remove the lid a