Step 1: Preheat Oven: Preheat the oven to 375F (190C). Lightly grease a 9x13 inch baking dish.
Step 2: Saut Vegetables: Heat olive oil in a large skillet over medium heat. Add chopped onion and celery. Saut until softened, about 5-7 minutes.
Step 3: Prepare Stuffing: In a large bowl, combine the stuffing mix and chicken broth. Stir to moisten, then add the sauted onion and celery. Mix well.
Step 4: Layer Ingredients: Spread the stuffing mixture evenly into the prepared baking dish. Arrange the chicken pieces over the stuffing.
Step 5: Prepare Sauce: In a separate bowl, whisk together the condensed cream of chicken soup, sour cream, milk, and black pepper until smooth.
Step 6: Pour Sauce: Pour the sauce evenly over the chicken and stuffing.
Step 7: Bake: Cover the baking dish with foil and bake for 30 minutes. Remove the foil and bake for an additional 10 minutes, or until the chicken is cooked through and the top is golden brown.
Step 8: Garnish and Serve: Let the casserole rest for 5-10 minutes before serving. Garnish with chopped fresh parsley, if desired.
Optional Tips: Substitute cream of mushroom soup for cream of chicken soup. Add a layer of frozen mixed vegetables for additional nutrients. For a crispier top, sprinkle breadcrumbs mixed with melted butter over the casserole during the last 10 minutes of baking.