Heat the oven to 350 degrees F. Cook and stir the meat, onion and celery in a large skillet until the meat is brown. Drain off the excess fat. Stir in the salt. Arrange the stuffing cubes in a greased baking pan, 9 X 9 X 2 or 11 3/4 X 7 1/2 X 1 3/4-inches and top with the meat mixture. Beat the milk, eggs, soup, and mustard together and pour over the meat. Then sprinkle with the cheese and bake uncovered until a knife inserted in the center comes out clean, about 30 to 40 minutes. Cool for 5 minutes and then cut into squares and serve.