Cut the bread into 1-inch cubes. Set aside at room temperature to dry for 8 hours.
Butter the bottom and sides of a 9-by-13 inch baking dish. Melt the 2 tablespoons of butter in a medium skillet over medium heat. Add the mushrooms and cook, stirring occasionally, until the juices are released and evaporate, about 5 minutes. Set aside.
Put the bread, ham, Cheddar and mushrooms in the baking dish. Toss to combine and spread evenly in the dish.
Whisk together the eggs, milk, dry mustard and salt and pepper in a medium mixing bowl. Pour the milk mixture over the bread mixture, and then press the bread down so that it is submerged. Cover the dish with plastic wrap and refrigerate overnight.
Preheat the oven to 350 degrees F. Bake the casserole, uncovered, until it is puffed and set all over, about 45 minutes. Serve hot.
Complete your breakfast casserole by pairing it with Starbucks® Veranda Blend.Complete your breakfast casserole by pairing it with Starbucks® Veranda Blend.