Step 1 Using a steamer, or a saucepan or wok with a steamer insert, steam the beans until they are tender but still crunchy, 7 to 10 minutes. Transfer to a bowl of ice water to stop the cooking. Drain and pat dry.
Step 2 Whisk together the olive oil, lemon juice, mustard, onion, garlic, tarragon, sugar and salt. Pour the vinaigrette over the beans and toss to coat.
Step 3Spoon the beans onto a serving platter. Sprinkle with freshly ground black pepper to taste.