Preheat the oven to 325 degrees F (165 degrees C). Lightly grease a 9×13-inch baking dish. Place pork sausage in a large, deep skillet. Cook over medium-high heat until evenly brown. Drain and set aside. Melt butter in a large saucepan over medium heat. Place celery and onions in saucepan; slowly cook and stir until…