Clean the mussels, open them by boiling them in salt water. Remove them, break off the one of the two shells which is empty, and pass the water through a fine sieve. Make an ordinary brown roux. Add a good spoonful of thick tomato purée, and, chopped together, one head of garlic, a slice of stale bread, and parsley. Season well, put the mussels in a saucepan with this mixture, stirring well, and cover with some of the water in which they have boiled. Cook a quarter of an hour.