Mix onion, celery leaves, poultry seasoning, and dried bread cubes. Add 1 can soup and milk. Cover and set in refrigerator overnight. Next day, mix meat and mustard. Add bread mixture. Mix it thoroughly. Turn into a 13x9x2 inch pan. (I prefer Pyrex.) Pat it down. Spread 1 can of diluted soup on top. Bake at 375 degrees for about 1 hour, or a little less. Cut this casserole in squares and garnish with parsley.