In a deep pan, melt the butter, add the onion and cook over medium heat until transparent and soft, stirring continuously to prevent colouring. Stir in the mussels, wine, bay leaf and thyme and place a tight- fitting lid on the pan. Turn the heat to high and cook rapidly for 2–3 minutes, shaking the pan periodically, until all the mussels have opened. Discard any mussels