Spray an 8 x 8-inch square glass or ceramic baking dish with cooking spray; set aside.
In a medium skillet over medium heat, melt butter and add mushrooms. Cook mushrooms about 5 minutes or until softened and brown at edges. Stir in green onion; set aside.
Place 1/2 of the bread cubes in prepared baking dish. Scatter 1/2 of the mushroom mixture and 1/2 of the cheese over bread cubes. Layer remaining bread cubes and mushroom mixture; set aside.
In a large bowl, beat milk, egg substitute, pepper sauce and salt, if desired, until well blended. Pour milk mixture over bread cubes and top with the remaining cheese. Make ahead suggestion: cover dish with foil and refrigerate for 8-10 hours before baking.
Preheat oven to 350 degrees Fahrenheit. Bake, covered for 45 minutes. Remove foil and bake an additional 15 minutes or until top is puffed up and cheese is browned at edges. Let cool for 5 minutes; cut into squares to serve.