In a deep pan melt the butter over a low-medium heat. Add the onion and garlic and cook until soft, but not browned. It should take around 8 minutes.

Squeeze the juice from the lemon half and add it in with the lemon. Add the mussels and wine, cover and steam until most of the mussels open.

Lower the heat to the lowest setting then pour in the cream, cover and steam until all the mussels are open. Scatter over the chopped herbs and serve with toasted bread.