This is a way of cooking mussels much appreciated in the south-west of France. Clean them well and put them on the fire in a saucepan with a glassful of water. When they are open, remove the empty half of the shells and pass the stock through a muslin. Make a brown roux, to which you add a tablespoonful of thick tomato purée, and, chopped together, a slice of stale bread, a clove of garlic, and parsley.
Season well, put the mussels in a saucepan with this sauce, add a little more of the stock, and cook a quarter of an hour.