Chicken and tarragon work in tandem to bring a summery feel to these picnic-ready sandwiches.
Combine the chicken, stock, tarragon sprigs, 1 teaspoon salt, 1/2 teaspoon pepper, and bay leaf in a large saucepan, and bring to a boil over medium-high heat. Reduce heat to medium-low. Simmer until chicken is cooked through, 12 to 14 minutes. Transfer chicken to a cutting board, and let cool completely.
Stir together the mayonnaise, mustard, lemon juice, and garlic. Stir in chopped herbs. Spread 3 tablespoons herb mayonnaise over cut sides of the ficelle.
Cut chicken crosswise on the diagonal into 1/8-inch-thick slices. Sprinkle chicken with salt and pepper. Lay chicken slices on bottom half of ficelle, overlapping slightly. Top with watercress.
Cut the sandwich on the diagonal into 4-inch pieces; discard ends. Wrap each sandwich in parchment paper.