Potatoes and Haricots Verts with Vinaigrette

If you're making your vinaigrette with white-wine vinegar, use 1/2 teaspoon sugar (instead of 1/4 teaspoon) to balance the higher acidity.

• 3 tablespoons Champagne vinegar or white-wine vinegar
• 1/2 teaspoon Dijon mustard
• 1/4 teaspoon black pepper
• 1/4 teaspoon sugar
• 2 teaspoons salt
• 1/2 cup olive oil
• 3 lb small (1 1/2- to 2-inch) yellow-fleshed potatoes such as Yukon Gold, scrubbed well
• 3/4 cup diced (1/4-inch) red onion
• 3/4 lb haricots verts or other thin green beans, trimmed and cut into 2-inch pieces
• 3 celery ribs, cut into 1/4-inch dice
• 1/3 cup finely chopped fresh flat-leaf parsley

Preparation

Whisk together vinegar, mustard, pepper, sugar, and 1 teaspoon salt in a small bowl. Add oil in a slow stream, whisking until emulsified.

Quarter potatoes, then cover with cold water by 1 inch in a 4- to 5-quart pot and bring to a boil with remaining teaspoon salt. Reduce heat and simmer, uncovered, until potatoes are tender, 8 to 10 minutes. Drain, then transfer hot potatoes to a large bowl and toss with onion and all but 1/4 cup vinaigrette. Cool to room temperature, about 1 hour.

While potatoes cool, cook green beans in a 3-quart saucepan of boiling salted water, uncovered, until crisp-tender, 3 to 4 minutes, then drain and transfer to a bowl of ice water to stop cooking. Let stand 2 minutes. Drain and pat dry.

Just before serving, toss potato mixture with green beans, celery, parsley, and remaining 1/4 cup vinaigrette.