In a 13 x 9 inch casserole dish, layer 6 slices of bread, 1 cup of cheese, and half of the diced ham. Then layer the remaining bread, cheese and ham.
In a small bowl, mix eggs, milk and mustard. Pour milk mixture all over casserole. Then pour on cream of mushroom soup.
Let it refrigerate overnight, or at least 8 hours.
Bake at 325°F (160°C) uncovered for 1 hour 15 minutes.