How to Make It
Place the potatoes in a large pot with enough cold water to cover and bring to a boil. Add 1 tablespoon salt, reduce heat, and simmer until tender, 15 to 18 minutes. Drain, run under cold water to cool, and cut in half.
Meanwhile, place the eggs in a large saucepan with enough cold water to cover. Bring to a boil. Remove from heat, cover the saucepan, and let stand for 12 minutes. Drain and run under cold water to cool. Peel and roughly chop.
In a large bowl, whisk together the mayonnaise, yogurt, lemon juice, mustard, shallots, ½ teaspoon salt, and ¼ teaspoon pepper until smooth.
Add the potatoes, eggs, watercress, and chives and toss to coat.