Place the mussels in the freezer for 10 minutes before steaming.

In a large covered steam kettle. bring the wine to a rolling boil over high heat. Place the mussels in the steam basket, cover, and cook just until they open. (Peek in after about 4 minutes.) Using tongs or a slotted spoon, begin transferring the open mussels to a large bowl. Discard any mussels-that do not open.

Strain the cooking liquid through a wire strainer lined with a double thickness of cheesecloth. and reserve. With a sharp paring knife, remove the mussels from the shells and reserve them in the strained juice.

In a large saucepan of lightly salted boiling water, cook the potatoes until tender when pierced with a knife, about 20 minutes. Drain and rinse under cold running water until cool enough to handle. Peel, cut into quarters lengthwise, then thinly slice. Place the potato slices in a large bowl. Add the scallions.

Lift the mussels out of the liquid and add to the potatoes. In a small bowl, whisk the oil and vinegar. pour over the potatoes and toss. Add just enough of the mussel cooking liquid to moisten as desired. Season with salt and pepper. Serve chilled or at room temperature. sprinkled with the optional truffle.