Cook green beans in boiling water for 6 minutes or until crisp-tender, and drain.
Combine the green beans and onion in a large bowl.
Heat a small saucepan over medium heat. Add vinegar, Dijon mustard, olive oil, salt, and black pepper; cook over medium heat 30 seconds or until thoroughly heated, stirring vinaigrette constantly with a whisk.
Drizzle vinaigrette over bean mixture; toss well to coat. Serve immediately.