1-1/2 lb. fingerling potatoes
1/4 cup extra-virgin olive oil; more to taste
Kosher salt and freshly ground black pepper
1/2 lb. haricots verts or slender green beans, trimmed
1-1/2 cups plain whole-milk Greek yogurt, such as Fage Total
1-1/2 Tbs. chopped fresh flat-leaf parsley
3 medium stalks celery, trimmed and sliced 1/8 inch thick on a sharp diagonal
1 English cucumber, peeled, cut in half lengthwise, seeded, and sliced 1/8 inch thick on the diagonal
1 medium bulb fennel (about 1 lb.), trimmed, cut in half, cored, and sliced crosswise 1/8 inch thick
1 small red onion, halved and sliced crosswise 1/8 inch thick
1 Tbs. fresh lemon juice; more to taste
Heat the oven to 400°F. On a small-rimmed baking sheet, toss the potatoes with 1 Tbs. of the oil and season generously with salt and pepper. Roast until tender when pierced with a skewer, 25 to 30 minutes. Cool the potatoes; they can be served still warm or at room temperature.
Meanwhile, bring a medium saucepan of salted water to a boil. Cook the haricots verts until just tender (taste one to see), 3 to 4 minutes. Drain in a colander and run under cold water until cool.
Combine the yogurt and parsley and season well with salt and pepper; keep chilled until ready to serve.
To serve, halve or quarter any larger fingerlings but leave the tiny ones whole. Combine the potatoes, haricots verts, celery, cucumber, fennel, and red onion in a large bowl and toss with the remaining 3 Tbs. oil and the lemon juice. Season well with salt and pepper and toss again. Taste and add more lemon juice or olive oil if needed.
Divide the salad among six to eight plates, piling it toward the center of each plate. Drop a large dollop of yogurt next to each salad.
Make Ahead Tips
You can prepare both the yogurt and the vegetables a few hours ahead of serving. Stir together the yogurt, parsley, and seasonings and refrigerate, covered. Combine all the vegetables but the potatoes in a bowl but don’t dress them. Cover the vegetables with a damp paper towel and keep refrigerated; leave the potatoes at room temperature.