Cut green beans into thirds. Cover and steam 10 minutes or until crisp but tender. Immediately plunge into cold water to stop the cooking process; drain and pat dry.
Mix together green beans, onion, cheese, and walnuts in a large bowl. Whisk together olive oil and next 5 ingredients. Note: this makes more dressing than you will need.
Pour vinaigrette over green bean mixture, and chill 1 hour; toss just before serving.