Place the potatoes in a pot with enough water to cover them. Add 1 tablespoon of salt, bring to a boil, and simmer for 15 to 30 minutes, depending on the size of the potatoes, just until tender when pierced with a small knife or skewer. Drain in a colander. Put a kitchen towel over the colander and allow the potatoes to steam for 10 minutes. When cool enough to handle, peel the potatoes and slice 1/2 inch thick. Place the potatoes in a mixing bowl. Meanwhile, combine the mayonnaise, lemon juice, vinegar, 2 teaspoons salt, and the pepper. While the potatoes are still warm, pour the dressing over the potatoes and toss well. Add the scallions, red onion, tarragon, and dill and toss gently. Allow the salad to sit for at least 30 minutes for the flavors to develop. Sprinkle with salt and serve at room temperature.