Prepare the chicken by shredding or dicing into small pieces.
In a large bowl, mix mayonnaise (or Greek yogurt), Dijon mustard, lemon juice, salt, and pepper until smooth.
Add chicken, celery, and red onion to the bowl. Stir well until everything is evenly coated.
Taste and adjust seasoning. Add optional mix-ins like grapes, apples, or nuts if desired.
Chill for at least 30 minutes before serving to let the flavors blend.
Serving Suggestions:
Serve in sandwiches with whole-grain bread.
Use as a filling for wraps with lettuce and tomato.
Spoon over a bed of mixed greens for a salad bowl.
Try lettuce wraps for a low-carb option.