Green Beans in Mustard Vinaigrette

1 1/2 pounds green beans, trimmed
1 cup vertically sliced red onion
3 tablespoons red wine vinegar
2 tablespoons Dijon mustard
1 tablespoon extravirgin olive oil
1/4 teaspoon salt
1/4 teaspoon black pepper

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.

from Cooking Light