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.
Whisk together the olive oil, lemon juice, mustard, onion, garlic, tarragon, sugar and salt. Pour the vinaigrette over the beans and toss to coat.
Spoon the beans onto a serving platter. Sprinkle with freshly ground black pepper to taste.
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