Wash green beans; remove ends and strings. In a large saucepan cook green beans and potatoes, covered, in a small amount of lightly salted boiling water for about 10 minutes or just until tender. Drain; place vegetables in a medium bowl. Cover and chill for 2 to 24 hours.
Prepare Nicoise Dressing. To serve, line 4 salad plates with lettuce leaves. Arrange chilled vegetables, tuna, tomatoes, eggs, and olives on the lettuce-lined plates. Sprinkle each serving with green onions. If desired, top each salad with an anchovy fillet. Shake dressing; drizzle over each salad.
In a screw-top jar combine oil, vinegar, honey, tarragon, mustard, salt, and black pepper. Cover and shake well.