In a large bowl, combine the diced chicken, mayonnaise, and Greek yogurt.
Add the chopped celery, red grapes, and green onions to the bowl.
Season the mixture with salt and pepper to taste.
Gently fold all ingredients together until well combined.
Lay lettuce leaves on the bread or rolls.
Spoon the chicken salad mixture generously onto the lettuce.
Top with additional greens or herbs if preferred.
Place the top slice of bread or roll over the filling.
Cut the sandwich in half for easier handling.