If using sous-vide: Generously season the chicken with salt and pepper. Thinly slice one lemon and arrange with whole tarragon sprigs alongside the chicken in zipper-lock or vacuum-seal bags. For zipper bags, expel the air using the water displacement method by sealing most of the bag, submerging it slowly into water, and sealing just before it fully submerges. For vacuum bags, seal as per the manufacturer’s guidance.

Preheat the sous-vide immersion circulator to 155°F (68°C). Submerge the sealed bags in the water bath and cook for a minimum of 1 hour and up to 4 hours. Once done, immediately transfer the chicken to an ice bath for 15 minutes to cool thoroughly.

While the chicken chills, extract 1 tablespoon of juice and 1 teaspoon of zest from the second lemon and combine in a large mixing bowl. Incorporate mayonnaise, mustard, minced tarragon, parsley, chives, red onion, celery, and garlic, mixing thoroughly with a rubber spatula. Taste and adjust with additional lemon juice if needed. Keep this mixture chilled until use.

Remove chicken from the bags, discarding the tarragon stems and lemon slices. Separate and discard the skin and bones. Dice the meat into 1/2-inch cubes and fold gently into the mayonnaise mixture until evenly combined. Taste and adjust seasoning with salt, pepper, and, if desired, additional mayonnaise, mustard, or lemon juice. Serve on a bed of lettuce or within a sandwich roll.