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
1 pound haricots verts (thin French green beans) or regular green beans
6 ears corn
Instructions
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.
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.
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.