Prepare the produce: Wash and dry the apple, celery, and lettuce. Finely chop the apple, thinly slice the celery, and shred the lettuce.
Dice the cheese: Cut the cheddar cheese slices into small, uniform pieces.
Mix the filling: In a medium bowl, combine the chopped apple, sliced celery, shredded lettuce, diced cheese, Caesar dressing, and chopped walnuts.
Stir until everything is evenly coated.
Chill for flavor: For the best taste, cover the bowl and refrigerate the filling for at least 2 hours. This allows the flavors to meld together beautifully.
Assemble the pitas: Cut the pita pocket in half to create two pockets. Gently open each half and stuff it with the chilled filling.
Serve immediately.