This light and bright chicken salad will wake up any lazy summer afternoon.
In a medium bowl, whisk the mayonnaise, half the shallots, the mustard, and 1 tsp. lemon zest. Fold in the chicken, 1/2 cup sliced celery, half the parsley, and half the tarragon. Season the chicken salad with salt and pepper.
In a small bowl, toss the remaining 1/4 cup sliced celery and the remaining shallots, parsley, and tarragon with the celery leaves and lemon juice. Season the celery salad.
In a large skillet, melt the butter over medium-high heat. Add the buns, split-side down. Cook until toasted, 3 to 5 minutes. Line the buns with lettuce. Top with the chicken salad, then the celery salad. Garnish with the remaining 1 tsp. lemon zest.