How to cook
Boil the mussels in salted water, drain them and braise in some of the oil. In the rest of the oil, braise the finely chopped onions.
Sprinkle on the flour and add the tomato paste that you've diluted with some of the water from the boiling of the mussels. Add the mussels and a little more of the water in which they were boiled.
Salt the dish and let it simmer on low heat until ready. Upon serving, sprinkle it generously with finely chopped parsley.