Coat 12 x 7 x 2-inch baking dish with cooking spray. Place 4 slices ham over bottom of dish. Top with 3 slices cheese, 4 slices bread. Layer with remaining ham, 3 slices cheese, 4 slices bread, 3 slices cheese.
Whisk eggs, milk, mustard, salt and pepper in bowl. Pour over strata in baking dish. Cover with plastic wrap. Chill 2 hours or up to 24 hours.
Heat oven to 350 degrees F. Remove wrap from baking dish; replace with foil.
Bake in 350 degree F oven 20 minutes. Uncover. Bake 25 minutes. Broil to crisp top, if desired. Serve warm. Makes 6 servings.