In a jar with a tight-fitting lid, add oil, vinegar, garlic, salt and pepper. Shake to emulsify. Set aside.
Place beans in a steamer basket and place over boiling water. Cover and allow to steam for 12 minutes or until crisp-tender. Immediately plunge beans into cold water to stop the cooking process. Drain and pat dry.
Combine beans, onions and cheese in a large bowl. Toss well. Pour oil mixture over top, cover and chill for 1 hour. Add walnuts, toss and serve.