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 10 to 15 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 (if desired) on the lettuce-lined plates. Sprinkle each serving with green onions. If desired, top each salad with an anchovy fillet and garnish with tarragon. Shake dressing; drizzle over each salad. Makes 4 main-dish servings.
In a screw-top jar combine olive or salad oil; white wine vinegar or white vinegar; honey; tarragon; Dijon-syle mustard; salt; and black pepper. Cover and shake well. Makes about 1/2 cup.