Bring large saucepan of lightly salted water to boil. Add quinoa and cook 15 min. or until tender. Drain, rinse under cold water and drain well. Bring large saucepan of lightly salted water to boil. Add green beans and cook just until tender, 6 min. Rinse under cold water; drain well.
Arrange lettuce leaves on 4 plates. Top each with quinoa, tuna, pimentos, olives and tomato wedges. Scatter with capers. Serve with Tarragon Vinaigrette.
Whisk together ¼ cup red wine vinegar, 2 tbsp. dijon mustard, ⅛ tsp. salt, and 2 tbsp. chopped fresh tarragon. Gradually whisk in ⅓ cup extra virgin olive oil until blended.