Directions

1. In a kettle, cover potatoes with salted cold water by 2 inches and simmer until just tender, about 25 minutes. Drain potatoes in a colander.

2. When potatoes are just cool enough to handle, halve larger ones and in a large bowl toss potatoes with 2 tablespoons vinegar.

3. In a small bowl whisk together mustard, remaining 3 tablespoons vinegar, and salt and pepper to taste. Add oil in a slow stream, whisking until emulsified.

4. Dressing may be made 1 day ahead and chilled, covered.

5. Bring dressing to room temperature before using.

6. Trim beans and, working over a bowl, cut corn kernels from cobs.

7. Have ready a bowl of ice with cold water. In a large saucepan of boiling salted water cook beans until crisp-tender, about 1 minute for haricots verts or about 5 minutes for regular string beans, and transfer with a slotted spoon to ice water to stop cooking.

8. Using a slotted spoon or skimmer, remove cooked beans and blot dry. Add to potatoes.

9. Return water in pan to a boil and blanch corn 30 seconds, or until crisp-tender. Drain corn in a sieve and rinse under cold water to stop cooking. Drain corn well and add to salad.

10. Salad may be prepared up to this point 1 day ahead and chilled, covered. Bring salad to room temperature before proceeding.

11. Gently toss salad with dressing and salt and pepper to taste until combined well.

12. Garnish edges of bowl with spears of greens before serving.