In a large salad bowl, whisk together the lemon juice, shallot, mustard and dried herbs. Whisk in the EVOO to combine.
Bring 2 inches of water to a boil in a small, high-sided skillet or saucepan. Salt the water, add the potatoes and green beans and cook for 4 minutes; add the bell pepper during the last minute of cooking. Using a slotted spoon, transfer the vegetables to a colander and rinse with cold water to stop the cooking; drain. Add the cooked vegetables to the salad bowl, along with the olives, lettuce, tomatoes, celery and onion. Add the tuna, eggs and tarragon and toss the salad to coat evenly with the dressing; season with pepper. To serve, pack the salad into the baguettes.