Let the rotisserie chicken cool to room temperature, then remove skin and bones and shred the meat into bite-sized pieces using two forks.
Finely dice the celery and red onion, halve the grapes, and toast the walnuts in a dry skillet over medium heat for 4–5 minutes until fragrant. Cool, then chop.
In a small bowl, whisk together the mayonnaise, Greek yogurt, lemon juice, tarragon, salt, and pepper until smooth and thick.
Add the celery, grapes, walnuts, and onion to the shredded chicken. Pour the dressing over the top and fold gently until evenly coated.
Cover and refrigerate the chicken salad for at least 30 minutes before serving.
Split the croissants, layer with butter lettuce, top with a generous scoop of chilled chicken salad, and close. Slice in half to serve.