1. Cook green beans in boiling salted water to cover 5 to 6 minutes or until crisp-tender; drain. Plunge green beans into ice water to stop the cooking process, and drain.
2. Stir together vinegar, brown sugar, and mustard until smooth.
3. Cook shallots in hot oil in skillet over medium heat, stirring often, 1 minute or until tender. Stir in vinegar mixture, salt, and pepper. Add green beans; toss to coat. Serve immediately.