Cook the Chicken: In a large saucepan, boil the chicken breasts in lightly salted water until fully cooked, about 30 minutes. Reserve the broth after cooking. Once cool, cut the chicken into bite-sized pieces and arrange them in a 9×13-inch baking dish.
Preheat Oven: Set your oven to 350°F (175°C).
Prepare Soup Mixture: In a medium bowl, mix together the condensed cream of chicken and cream of celery soups. Use an empty soup can to measure 10 ½ fluid ounces of milk, and stir it into the soups until smooth. Pour this mixture over the chicken in the baking dish.
Moisten Stuffing: In a separate bowl, combine the cornbread stuffing mix with chicken broth, ensuring the stuffing is evenly moistened. If using, mix in diced onion and chopped celery.
Assemble Casserole: Spread the stuffing mixture evenly over the soup-covered chicken in the baking dish.
Bake: Place the casserole in the preheated oven and bake for about 45 minutes, or until the dish is thoroughly heated and the top is golden brown.
Serve: Allow the casserole to cool slightly before serving to let the flavors meld.
