Remove leaves from celery, cut stalks into 5-inch pieces and wash well. Place in 2 quarts boiling water with onion, salt pork and salt; boil 15 minutes, or until celery is tender. Drain. Place 1 layer cooked celery in greased casserole, sprinkle abundantly with cheese and with butter, and repeat layers until all celery is used, ending with cheese. Bake in 400 degrees hot oven for 7 minutes. Serves 4.