Scrub and debeard the mussels.
Heat the olive oil in a wide deep skillet over high heat. Add the mussels, cover, and let steam open, 3 to 4 minutes. Let cool, then remove and discard the top shells, leaving the mussels attached to their remaining half-shells. Place on a serving platter.
To make the vinaigrette, put the shallot in a small bowl. Add the red wine vinegar and a good