Drain the potatoes thoroughly in a colander, return them to the pan, and steam over medium heat, shaking the pan, or until any excess liquid is evaporated, about 30 seconds. Put the potatoes through a ricer or mash with a potato masher while still hot. Stir half the vinaigrette into the potatoes along with:
8 slender scallions, very thinly sliced
1/4 cup minced fresh parsley
Lots of salt and ground black pepper to taste
Spoon the potatoes on a platter, make a lengthwise valley down the center, and fill the valley with the remaining vinaigrette. Garnish the whole platter with: