Pull all the crumb from the rolls. Butter the insides. Stir the hot mussels into the hot sauce and fill the rolls. Butter the outsides of the rolls and stand in the oven at 300° F., gas mark 2, for 10 minutes. Serve with sprigs of watercress.
If you want to prepare the rolls ahead of the meal, proceed as above but wrap each in foil and leave in the oven for 20 minutes to ensure their be