* 1/4 cup Champagne vinegar
* 2 teaspoons Dijon mustard
* 2 large garlic cloves, minced
* 1/2 teaspoon sugar
* 1/3 cup extra-virgin olive oil
* 1 pound green beans, trimmed and halved
* 1 pound small (1- to 2-inch) yellow-fleshed potatoes such as Yukon Gold
* 2 (6-ounce) cans tuna in olive oil, drained
* 12 ounces cherry tomatoes, halved
* 1/2 cup pitted Kalamata olives (3 ounces)
* 3 tablespoons rinsed drained bottled capers (1 ounces)
* 1/2 cup finely chopped flat-leaf parsley
* 4 hard-boiled large eggs, quartered
Whisk together vinegar, mustard, garlic, sugar, 1/2 teaspoon salt, and 1/4teaspoon pepper, then add oil in a slow stream, whisking until blended.
Cook beans in a pot of boiling salted water (2 tablespoons salt for 6 quarts water), uncovered, until tender, 5 to 6 minutes. Immediately transfer with a slotted spoon to a bowl of ice water to stop cooking. Drain beans and pat dry.
Add potatoes to boiling water and simmer, uncovered, until tender, about 20 minutes, then drain. Halve potatoes while still warm.
Gently flake tuna and toss with 1 tablespoon dressing.
Toss potatoes and beans with tomatoes, olives, capers, parsley, and remaining dressing in a large bowl. Season with salt and pepper and top with tuna and eggs.