Shred the chicken: Remove skin and bones from a rotisserie chicken, then shred the meat into bite-sized pieces. You should have about 3 cups.
Prep the vegetables: Finely dice the celery and red onion. Chop the parsley.
Make the dressing: In a large bowl, whisk together mayonnaise, Dijon mustard, lemon juice, salt, and pepper.
Combine: Add the shredded chicken, celery, onion, and parsley to the dressing. Stir until everything is evenly coated.
Adjust seasoning: Taste and add more salt, pepper, or lemon juice if needed.
Chill (optional but recommended): Cover and refrigerate for at least 30 minutes to let flavors meld.
Serve: Spoon onto rolls, croissants, or lettuce cups. Garnish with extra parsley if desired.