In a large mixing bowl, combine the diced or shredded cooked chicken breast, chopped celery, chopped red onion, grapes (if using), dried cranberries or raisins (if using), and chopped pecans or almonds (if using).
In a separate small bowl, whisk together the mayonnaise, Greek yogurt, lemon juice, Dijon mustard, salt, and pepper until smooth and well combined.
Pour the dressing mixture over the chicken and other ingredients in the large mixing bowl. Stir gently to coat everything evenly with the dressing.
Taste the chicken salad and adjust the seasoning as needed with additional salt, pepper, or lemon juice.
Cover the chicken salad and refrigerate it for at least 30 minutes to allow the flavors to meld together and the salad to chill.
Before serving, garnish the chicken salad with fresh herbs (if using) for a pop of color and freshness.
Serve the chicken salad on its own, or as a sandwich filling, wrap filling, or atop a bed of mixed greens for a light and refreshing meal.