Bring a large pan of salted water to a boil, add the shrimp, reduce the heat, and simmer for 2 to 3 minutes, just until the shrimp are opaque throughout; don't overcook or the shrimp will be tough. Drain, dry on paper towels, and chill until you're ready to assemble the salad (you can do this up to 1 day ahead).
Put the potatoes in a large saucepan, cover with well-salted water, and bring to a boil. Reduce the heat and gently boil until tender, 8 to 15 minutes, depending on size and variety of potato. Test by cutting and tasting a chunk to be sure it's fully cooked.
Drain the potatoes, and while they're still hot, add about three-quarters of the vinaigrette; toss to coat well. Let the potatoes stand for about 30 minutes so the dressing is absorbed.
In a small saucepan, bring a few cups of salted water to a boil. Add the beans and boil for 1 minute. Add the peas; continue boiling for another 2 minutes, just until both vegetables are crisp-tender. Drain and immediately rinse in cold water to stop the cooking; drain very well.
When you're ready to serve the salad, toss the potatoes with the shrimp, peas, beans, celery, radishes, and scallions, and the remaining vinaigrette. Season well with salt and pepper. Arrange on plates with a bit of watercress, if you like, and the lemon zest and chives.
nutrition information (per serving):
29, Fat Calories
260, Saturated Fat
21, Monounsaturated Fat
41, Polyunsaturated Fat
Photo: Martha Holmberg