Combine the lemon juice, mustard, celery salt, and 1/4 teaspoon pepper in a small bowl, using a whisk or a fork. Gradually add the olive oil, continuing to whisk until fully incorporated. Combine the green beans, tuna, olives, bell pepper, onion, and tarragon in a medium bowl. Pour the vinaigrette over the salad mixture and gently toss to coat the vegetables.