Place in a large sauce pan:
      6 large baking potatoes, peeled and cut into 1-inch chunks
Add cold salted water just to cover. Bring to a boil and cook the potatoes until tender, about 10 minutes. Meanwhile, whisk together in a small bowl:
      1/2 cup sour cream or yogurt
      2 tablespoons apple cider vinegar
      1 1/2 teaspoons Dijon mustard
      1 teaspoon minced fresh rosemary, or scant 1/2 teaspoon dried rosemary
      Salt and ground black pepper to taste
Gradually whisk in to emulsify the vinaigrette:
      1/2 cup olive oil
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:
      Minced fresh parsley
Serve warm or at room temperature.