Heat the butter in a pan until it foams. Add the diced bacon and cook over a moderate heat for 5-8 minutes until lightly browned.
Stir in the garlic, onion and thyme and fry for a further 5 minutes until the onion becomes translucent.
Discard any open mussels, then add the remainder to the pan with the Calvados and diced apple. Cover and leave to steam for a few minutes until the mussels start to open. Pour in the heavy cream and gently bring up to the boil. Season.