In a large mixing bowl, carefully combine the 2 cups of cooked chicken, 1 diced avocado, 1/2 cup of finely chopped celery, 1/4 cup of finely chopped red onion, 1/4 cup of chopped fresh parsley, and 2 tablespoons of chopped fresh dill. Gently mix everything together until evenly distributed, about 2 minutes.

In a small bowl, whisk together 3 tablespoons of Greek yogurt, 1 tablespoon of fresh lemon juice, 1 teaspoon of Dijon mustard, and 1/4 teaspoon of garlic powder. Season with salt and black pepper to taste, whisking until smooth, about 1 minute.

Pour the prepared dressing over the chicken mixture in the large bowl. Gently toss everything together with a spatula, ensuring each ingredient is coated with dressing, about 1-2 minutes.

To serve, take 8 large butter lettuce leaves and spoon the chicken salad mixture into each leaf. Garnish with optional halved cherry tomatoes.