Biscuits and Gravy Casserole

Name: Biscuits and Gravy Casserole
Ingredients: 2 1/4 cups (315 g) all-purpose flour (see Recipe Note); 3 tablespoons cornstarch; 1 tablespoon baking powder; 3/4 teaspoon kosher salt; 1/2 cup unsalted butter , cold; 1 cup buttermilk , cold; 1 tablespoon olive oil; 1/2 pound sliced mushrooms; 1/4 cup chopped yellow onions; 1 pound breakfast pork sausage; 6 tablespoons all-purpose flour; 1/2 teaspoon ground sage; 1/2 teaspoon freshly ground black pepper; 1/8 teaspoon ground nutmeg; 2 1/2 cups milk , preferably whole; 1 tablespoon unsalted butter , melted