Don't stop at beans. You'll want to use the lemony vinaigrette from this recipe all summer long on lettuce salads, asparagus or boiled baby potatoes.
Fill a large bowl with ice water and set aside. Bring a large pot of salted water to boil. Add all beans and cook until crisp-tender, 3 to 5 minutes. Drain. Immediately plunge drained beans into the bowl of ice water to stop the cooking. Once beans are cool, drain and allow to dry completely.
In a large bowl, whisk together shallot, lemon juice, tarragon, mustard, salt, sugar and pepper. While whisking, slowly pour in olive oil and continue whisking until emulsified. Add beans; toss to coat.
Dressed green beans can lose their color after two or three hours. If your salad will be sitting longer than that (and you care about looks), transport the dressing separately.