1 Preheat the oven to 400°F.
2 Make the biscuit dough: Combine the flour, cornstarch, baking powder, and salt in a large bowl. Cut the butter into 1/2-inch chunks and sprinkle it over the dry ingredients. Using a pastry blender, a butter knife, or a fork, cut the butter into the dry ingredients until most of it is the size of peas. Don’t overwork it; you want to leave some of the butter in chunks.
Add the buttermilk and toss the mixture with a spatula until it starts to look shaggy, and most of the buttermilk has been absorbed. It won’t form a dough yet; that’s OK.
3 Bring the dough together: Turn out the contents of the bowl onto a clean surface dusted with a little flour. Gather the dough together with your hands into a mound, then roll it out until it is about 10-inches wide. Again, don’t worry if the dough is crumbly!
4 Fold the dough to create flaky layers: Use a spatula (like the kind you use to make fried eggs) or bench scraper and flip 1/3 of the dough over from the left to the center. Then flip the other side over that side. You’re basically folding the dough over like a letter.
Rotate the dough 90-degrees and roll it out again to about a 10-inch square. It will start to feel more like a dough and less crumbly at this point, but there will still be some loose chunks. Just press them back into the dough if they fall out, and resist the urge to add more liquid.
Repeat the process of rolling out and folding two more times. By the fourth time, the biscuit dough will hold together and feel like actual dough.
5 Cut the biscuits: Roll out the dough to a 10-inch square that’s roughly 1/2 inch thick. Using a 2 1/2-inch biscuit cutter, cut out 16 circles. When you press down to cut out the dough, don’t twist the biscuit cutter, which will “seal” the edges of the dough. Instead, just cut straight down, so the biscuit will rise and be as flaky as possible.
5 Bake the bottom layer of biscuits: Set aside 12 of the biscuit rounds. Place the remaining 4 biscuit rounds on the bottom of a 2-quart glass or ceramic casserole dish. Cut up the remaining scraps of dough and jigsaw-puzzle the remaining dough into the bottom of the casserole dish, making sure to cover the bottom in a single layer.
Place on a rimmed baking sheet (for ease of removal from the oven and to catch any drips). Bake in the oven for 15 minutes or until the top of the biscuits start to look dry but not brown at all. (There might be a slight darkening around the edges, but don’t be alarmed if the biscuits still look pale white.)
6 Meanwhile, make the gravy: While the bottom biscuits are baking, make the gravy by placing the oil, mushrooms, and onions in a large sauté pan. Cook on high heat, stirring constantly, until the mushrooms start to soften, and the onions start to look translucent, about 5 minutes. Add the sausage, breaking it up with a wooden spoon or spatula, and cook until the sausage starts to brown, about 5 minutes.
Keep the heat on high, and add the flour, sage, black pepper, nutmeg, and salt to the pan. Stir until the ingredients are absorbed. Pour in the milk, reduce heat to medium, and cook, continuing to stir until the liquid has started to bubble and thicken, about 3 to 7 minutes. You’re now ready to assemble the casserole!
If the first layer of biscuits isn’t done baking, lower the heat on the gravy to a simmer until the biscuits are done. If you still need time to finish the gravy, but the biscuits are done—remove the casserole dish from the oven, and let it sit on the counter or stovetop while you finish the gravy.
7 Assemble and bake the casserole: Once the gravy has thickened and the first layer of biscuits is done baking, pour or spoon the gravy over the biscuit layer in the casserole dish. Place the 12 reserved biscuits over the gravy and brush the top of the biscuits with the melted butter.
Return the dish to the oven and bake until the biscuits are golden brown and the gravy is bubbling and thick, about 18 to 23 minutes.
8 Cool and serve: Let cool 10 minutes before serving warm from the casserole dish.