Cook green beans in boiling salted water to cover 8 minutes or until crisp-tender. Drain and plunge into ice water to stop the cooking process; drain and pat dry. Place in a serving bowl; cover and chill at least 2 hours.
Add chopped onion and Lemon Vinaigrette to beans, tossing to coat. Sprinkle with feta and walnuts. Garnish, if desired.