Put the well-scrubbed mussels in a pot. Pour in around 8 1/3 cups (2 l) cold water + 1 tsp salt and boil it on high heat until the shells open up and meat softens. Drain them, remove them from their shells, clean and distribute into two plates - from 4.5 lb (2 kg) of shells you get out about 17.5 oz (500 g) of meat.
The sauce is prepared as follows: In a bowl put the squeezed bread, previously soaked in cold water, the crushed garlic, oil, vinegar, salt and the broth from the mussels, strained through a cheesecloth to remove the sand, if any.
Pour the sauce over the mussels. They can be eaten as an appetizer, or as the main course.