ham breakfast casserole

One of my very favorite breakfast casseroles that I first tried at a mom's group. 

ham breakfast casserole (my friend Emily B.)
12 slices white bread
4 cups ham, cubed
1 cup grated sharp cheese
5 eggs
2 cups milk
1 teaspoon prepared mustard

sauce
1 can cream of mushroom soup
1 cup sour cream
1 cup sharp cheddar cheese
3 ounce can sliced mushrooms, optional

Trim bread crusts; butter bread.  Place 6 slices butter side down in 9x13 pan.  Place half of ham over bread then half of cheese.  Place remaining 6 slices of bread butter side up.  Cover with remaining ham and cheese.  Beat eggs, milk and mustard and pour over layers.  Refrigerate overnight.  Bake at 325 degrees for 40 minutes.  Add sauce and bake 20 minutes more.  Garnish with paprika.