Chicken Stuffing Casserole

Name: Chicken Stuffing Casserole
Ingredients: 5 tablespoons unsalted butter , divided; 7 ounces day-old sourdough loaf, cut in 3/4- to 1-inch cubes (about 4 cups); 1/2 teaspoon kosher salt , divided; 1/2 teaspoon freshly ground black pepper , divided; 3 stalks celery , halved lengthwise and cut into 1/2-inch pieces (about 1 1/2 cups ); 1 medium yellow onion , chopped (about 1 1/4 cups ); 1 tablespoon fresh rosemary , minced; 2 teaspoons fresh sage leaves , minced; 1 1/4 teaspoons fresh thyme , minced; 1/2 cup low-sodium chicken broth