Heat the olive oil and butter in a large heavy-based pan over medium heat. Add the leek and celery and cook, stirring occasionally, for 6–7 minutes until the vegetables are soft. Add the
garlic and cook, stirring, for 1 minute more.
Add the mussels and stir together gently until well combined. Add the wine and thyme sprigs, cover the pan and simmer for 6–7 minutes. Stir in the cream, cover and simmer for a further 5 minutes or until the mussels are open (discard any that haven’t opened in that time).
Serve with lots of crusty bread to soak up the sauce.