Cut green beans into thirds, and arrange in a steamer basket over boiling water. Cover and steam 15 minutes or until crisp-tender. Immediately plunge into cold water to stop the cooking process; drain and pat dry.
Toss together green beans, onion, cheese, and walnuts in a large bowl. Cover and chill 1 hour.
Whisk together olive oil and next 5 ingredients; cover and chill 1 hour.
Pour vinaigrette over green bean mixture, and chill 1 hour; toss just before serving.