In a large bowl, whisk together mayonnaise, greek yogurt, dijon mustard, and fresh lemon juice until completely smooth.
Season the dressing with salt and black pepper. Give it a taste and adjust to your liking.
Add the diced cooked chicken breast to the bowl and toss until every piece is evenly coated in the dressing.
Fold in the halved red grapes, diced celery, and chopped fresh tarragon. Mix gently so the grapes stay intact.
Sprinkle in the toasted pecans and fold them through the salad just until combined.
Cover the bowl and refrigerate for at least 30 minutes to let the flavors meld together. Serve chilled on its own, in a croissant, or over a bed of greens.