Cook the potato in boiling water for 6 minutes or until tender; remove with a slotted spoon. Add green beans to boiling water, and cook 3 minutes or until crisp-tender. Drain.
Prepare grill or broiler.
Place the fish on a grill rack or broiler pan coated with cooking spray; cook for 3 minutes on each side or until desired degree of doneness. Cut the fish into 1-inch chunks.
Combine potato, fish, onion, parsley, and tarragon in a large bowl. Combine the broth and next 5 ingredients (broth through pepper); stir well with a whisk. Pour 1/2 cup broth mixture over potato mixture, and toss well.
Divide beans, greens, and tomatoes evenly among 4 plates. Top each serving with 1 1/2 cups potato mixture and 1 tablespoon olives. Drizzle 1 tablespoon of remaining broth mixture over each serving.