Ingredients

Serves 12

　　4 pounds small white boiling potatoes (about 2 inches in diameter)

　　5 tablespoons cider vinegar

　　3 tablespoons coarse-grained mustard

　　cup plus 2 tablespoons olive oil, or to taste

　　1 pound haricots verts (thin French green beans) or regular green beans

　　6 ears corn

　　

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

　　

Step 2

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. Dressing may be made 1 day ahead and chilled, covered. Bring dressing to room temperature before using.

　　

Step 3

Trim beans and, working over a bowl, cut corn kernels from cobs. Have ready a bowl of ice and 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 green beans, and transfer with a slotted spoon to ice water to stop cooking. Drain beans well and add to potatoes. 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. Salad may be prepared up to this point 1 day ahead and chilled, covered. Bring salad to room temperature before proceeding. Gently toss salad with dressing and salt and pepper to taste until combined well.