Boil the chicken breast, cool, and shred it finely.
Prepare cranberries, chopped celery, and chopped onion.
Make the dressing by mixing Greek yogurt, whole grain mustard, and black pepper.
Mix the shredded chicken breast and prepared ingredients together with the dressing.
Place the mixed filling on two slices of whole wheat bread.
Add lettuce, top with bread, and finish the sandwich.