Can be prepared in 45 minutes or less.
In a small bowl, whisk together vinegars, soy sauce, garlic, gingerroot, and sesame oil. Add canola or vegetable oil in a stream, whisking, and whisk until emulsified. Season dressing with salt and pepper.
In a small dry skillet toast sesame seeds over moderate heat, stirring, until golden and transfer to a small bowl.
In a large saucepan of boiling salted water, cook beans until crisp-tender, about 4 minutes. Transfer beans to a colander and rinse under cold water. Drain beans well.
In a large bowl, combine beans, scallions, sesame seeds, and dressing and toss well. Serve salad chilled or at room temperature.