Make the vinaigrette ahead.
Bring about 3/4″ of salted water to a boil in a skillet. Add green beans, cover, and simmer for 5 to 6 minutes until tender, but still slightly crisp. Drain and plunge into ice water or add to a colander and place under cold, running water to preserve color and texture. Drain well when cooled.
Arrange tuna, white beans, green beans, and tomatoes on a platter. Drizzle with dressing. Serve with lemon. Garnish with herbs used in the vinaigrette, if desired.
Keywords: Canned Beans, Dried Beans, Fish and Seafood, French, Green Beans, leftover fish, Lemon, Main dish salad, Salad, Tomatoes, Tuna, Tuna Salad