Brown ground beef in skillet, stirring until crumbly; drain. Combine baking mix and milk in a bowl, mixing to form a dough. Grease bottom and sides of cold 12 to 14-inch electric skillet. Press dough over bottom and sides of prepared skillet. Layer tomato sauce, ground beef, oregano, mushrooms and cheese over dough. Set temperature to 325 degrees. Bake covered for 20 minutes or until crust is brown