Melt the butter in a deep saucepan and add the bacon cubes. Cook over a medium to high heat for 5 minutes.
While the bacon is cooking, de-beard the mussels. Wash the leeks and slice into thin rings. Place the mussels, leeks, thyme and cider in a bowl.
Once the bacon is crisp and golden brown, drain any excess fat from the pan. Return the bacon to the heat and add the bowl of mussels, leeks and cider. Cover with a tight fitting lid. Shake the pan and cook over a high heat for 2 to 3 minutes. The mussels should open and be plump and juicy after this time. Cooking them for longer will cause them to become tough.
Serve the mussels in a large bowl, discarding any that have not opened in the cooking process. You should not need to season this dish as the bacon and mussels are quite salty.
«De-beard your mussels just before you are ready to cook them rather than preparing them in advance. Make sure to discard any that do not open during the cook process - never force open a mussel.»