Lightly grease a 9x13 inch baking pan. Line pan with slices of bread.
Place slices of cheese over bread. Sprinkle diced ham over cheese. Pour melted butter over ham, cheese and bread. Beat together eggs, salt, mustard and milk; pour over bread slices.
Cover pan and refrigerate overnight. Bake in preheat oven at 350 degrees F (175 degrees C) for one hour or until set. Serve warm.
