In a medium skillet over medium heat, melt the butter. Add mushrooms and cook until tender, about 5 minutes.
Place half of the bread in a lightly sprayed or oiled 9-by-13-inch baking dish. Add sausage, mushrooms, half of the cheese, and remaining bread.
In a large bowl, combine the eggs, milk, salt, pepper, and mustard. Pour over the bread mixture and top with remaining cheese.
Cover with plastic wrap flush with the surface, and weight down with a glass dish on top to push the bread mixture to the bottom of the pan. Refrigerate overnight.
Let the dish sit out at room temperature for about 30 minutes before baking. Preheat the oven to 350 degrees. Bake, uncovered, until set and bubbly, about 45 to 50 minutes. Let stand for 10 minutes, then cut into squares and serve.