Heat the oven to 350 degrees. 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 9x9x2 inch baking pan 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.