Prepare the chicken: Chop or shred the cooked chicken into small, bite-sized pieces.
Chop the vegetables: Finely chop the celery and slice the green onions. Chop the parsley if using.
Make the dressing: Add the Greek yogurt, mayonnaise, Dijon mustard, lemon juice, and garlic powder to a small bowl.
Season the dressing: Add salt and black pepper and stir until smooth.
Combine the main ingredients: Place the chicken, celery, green onion, and parsley in a large bowl.
Add the dressing: Pour the yogurt mixture over the chicken.
Mix gently: Stir until the chicken and vegetables are evenly coated while keeping some texture.
Taste and adjust: Add more lemon juice, mustard, salt, or pepper if needed.
Chill the salad: Cover and refrigerate for about 30 minutes for the flavors to blend.
Serve: Stir and serve over greens, in lettuce cups, or with bread.