Cook the Chicken: Place chicken breasts in a large saucepan, cover with lightly salted water, and bring to a boil. Cook until thoroughly done, about 30 minutes. Reserve the cooking broth and set the chicken aside to cool slightly.

Prepare the Chicken and Oven: Cut the cooked chicken into bite-sized pieces and spread evenly in a 9x13-inch baking dish. Preheat the oven to 350°F (175°C).

Make the Creamy Sauce: In a medium bowl, mix the condensed cream of celery soup, condensed cream of chicken soup, and milk until smooth. Pour the mixture over the chicken in the baking dish.

Prepare the Stuffing: In a small bowl, combine the seasoned cornbread stuffing mix with 1 ½ cups of chicken broth (use the reserved broth). Stir until the stuffing is moistened. Add diced onion and chopped celery if using.

Assemble the Casserole: Spoon the stuffing mixture evenly over the chicken and soup mixture in the baking dish.

Bake the Casserole: Bake in the preheated oven for 45 minutes, or until heated through and the top is golden brown and slightly crispy.

Serve: Let the casserole cool slightly before serving. Enjoy warm!