In a medium bowl, combine shredded chicken, diced celery, and chopped red onion.
In a small bowl, whisk together Greek yogurt (or mayo), Dijon mustard, lemon juice, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and stir until everything is evenly coated. Taste and adjust seasoning if needed.
Cover the bowl and refrigerate for at least 15 minutes to let flavors meld.
Warm tortillas in a dry skillet over medium heat for about 30 seconds per side to make them pliable.
Lay a tortilla flat. If using lettuce, place a few leaves down the center. Spoon about 1/2 cup of chicken salad onto the tortilla.
Fold in the sides, then roll tightly from the bottom up. Slice in half diagonally and serve immediately.