In a small bowl, whisk mayonnaise, lemon juice, Dijon mustard, tarragon, salt, and pepper until smooth.

Toast walnuts in a dry skillet over medium heat, stirring often, for 3 to 4 minutes until fragrant. Cool and chop.

In a large bowl, combine shredded chicken, celery, red onion, and toasted walnuts. Toss lightly.

Pour the dressing over the chicken mixture and fold gently until everything is evenly coated.

Serve on croissants or lettuce leaves, topped with a crack of black pepper and extra tarragon if desired.