Heat 1 tablespoon oil in a heavy skillet over moderate heat until hot but not smoking, then cook mustard seeds, stirring, until they pop and are 1 shade darker, about 2 minutes. Transfer oil with seeds to a large bowl.
Simmer vinegar and sugar in a small saucepan, stirring, until sugar is dissolved, then cool 5 minutes.
Heat remaining 2 tablespoons oil in cleaned skillet over moderately high heat until hot but not smoking, then cook onion, stirring, until golden brown, 8 to 10 minutes. Remove from heat and stir in vinegar, then add to mustard oil in large bowl.
Have ready a large bowl of ice and cold water.
Cook beans in a pot of boiling salted water until crisp-tender, about 5 minutes. Drain in a colander and plunge into ice water to stop cooking, then drain well.
Toss beans with vinaigrette and salt and pepper, to taste. Serve at room temperature or chilled.
Recipe courtesy of Gourmet Magazine