Mandarin oranges, toasted almonds, and celery enliven chicken salad with bursts of wonderful color and flavor.
Bring 2 cups water, chopped celery, onion, carrot, thyme, and salt to a boil in a medium saucepan. Reduce heat to a simmer. Add chicken; cover and simmer until cooked through, about 15 minutes. Transfer chicken to a bowl; let cool, then shred with 2 forks. Discard contents of pan.
Stir together mayonnaise, lemon juice, and mustard in a medium bowl. Season with salt and pepper. Stir in chicken, sliced celery, and almonds. Gently fold in orange segments.
Cut 1 inch from top of each pita to open pocket. Fill each pita with 1 lettuce leaf and 1/2 cup chicken salad.