In a large mixing bowl, combine the mayonnaise, Greek yogurt (if using), Dijon mustard, sweet pickle relish, and fresh lemon juice. Mix well until the dressing is smooth and well-combined.
Add the shredded or diced cooked chicken to the bowl. Add the finely chopped celery and red onion as well. Season with salt and pepper to taste.
Gently fold all the ingredients together, ensuring that the chicken is evenly coated with the dressing and the vegetables are well-distributed.
Taste the chicken salad and adjust the seasoning or dressing as desired. You can add more lemon juice, mayo, or mustard based on your personal preference.
To serve, line a plate or bowl with lettuce leaves. Spoon the chicken salad onto the lettuce leaves, creating an attractive presentation.
If desired, you can also make sandwiches by spreading the chicken salad between slices of bread or croissants. Add sliced tomatoes, cucumbers, or avocados for extra freshness and texture.
Serve the chicken salad immediately or refrigerate for a few hours to allow the flavors to meld together.