Place the potatoes and 2 tablespoons salt in a large pot of water. Bring the water to a boil, then lower the heat and simmer for 10 to 15 minutes, until the potatoes are barely tender when pierced with a knife. Drain the potatoes in a colander, then place the colander with the potatoes over the empty pot off the heat and cover with a clean, dry kitchen towel. Leave the potatoes to steam for 15 to 20 minutes, until tender but firm.
Meanwhile, in a small bowl, whisk together the wine, chicken stock, lemon juice, garlic, mustard, 2 teaspoons salt and 3/4 teaspoon pepper. Slowly whisk in the olive oil to make an emulsion. Set aside.
When the potatoes are cool enough to handle, cut them into quarters or halves, depending on their size. Place the cut potatoes in a large bowl and pour enough dressing over them to moisten. (As the salad sits, you may need to add more dressing.) Add the onion, tarragon and parsley, and salt and pepper to taste. Toss well, cover, and refrigerate for a few hours to allow the flavors to blend. Serve cold or at room temperature.