1. Preheat oven to 400°F. Combine onion, zucchini and bell pepper. Toss with olive oil and basil.
2. Spread mixture in single layer on shallow baking sheet or roasting pan. Roast for 20 minutes or until vegetables are tender, adding green beans during last 5 minutes of roasting.
3. Drizzle with vinegar. Serve warm or chilled on lettuce leaf.