Prepare the Chicken: If using raw chicken breasts, season them with salt and pepper. Heat a skillet over medium heat and add a little oil. Cook the chicken breasts for about 8-10 minutes on each side, or until they are cooked through and no longer pink in the center. Once cooked, let them cool slightly, then shred the chicken using two forks.
Make the Dressing: In a small bowl, whisk together the mayonnaise, Greek yogurt, Dijon mustard, lemon juice, garlic powder, onion powder, salt, and pepper until well combined. Adjust seasoning according to taste.
Combine Ingredients: In a large mixing bowl, combine the shredded chicken, diced celery, chopped red onion, chopped parsley, and walnuts (if using). Pour the dressing over the chicken mixture and toss until everything is evenly coated with the dressing.
Chill and Serve: Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld together. When ready to serve, line a serving platter or individual plates with lettuce leaves. Spoon the chicken salad onto the lettuce leaves and serve chilled.