1 Preheat the oven to 400°F.
2 Make the biscuit dough: Mix the flour, cornstarch, baking powder, and salt in a big bowl. Reduce the butter into 1/2-inch chunks and sprinkle it over the dry elements. Utilizing a pastry blender, a butter knife, or a fork, minimize the butter into the dry elements till most of it’s the measurement of peas. Don’t overwork it; you wish to depart a few of the butter in chunks.
Add the buttermilk and toss the combination with a spatula till it begins to look shaggy, and many of the buttermilk has been absorbed. It gained’t type a dough but; that’s OK.
three Deliver the dough collectively: End up the contents of the bowl onto a clear floor dusted with slightly flour. Collect the dough collectively together with your arms right into a mound, then roll it out till it’s about 10-inches large. Once more, don’t fear if the dough is crumbly!
four Fold the dough to create flaky layers: Use a spatula (like the sort you employ to make fried eggs) or bench scraper and flip 1/three of the dough over from the left to the middle. Then flip the opposite facet over that facet. You’re principally folding the dough over like a letter.
Rotate the dough 90-degrees and roll it out once more to a couple of 10-inch sq.. It would begin to really feel extra like a dough and fewer crumbly at this level, however there’ll nonetheless be some free chunks. Simply press them again into the dough in the event that they fall out, and resist the urge so as to add extra liquid.
Repeat the method of rolling out and folding two extra occasions. By the fourth time, the biscuit dough will maintain collectively and really feel like precise dough.
5 Reduce the biscuits: Roll out the dough to a 10-inch sq. that’s roughly 1/2 inch thick. Utilizing a 2 1/2-inch biscuit cutter, minimize out 16 circles. While you press down to chop out the dough, don’t twist the biscuit cutter, which is able to “seal” the perimeters of the dough. As an alternative, simply minimize straight down, so the biscuit will rise and be as flaky as attainable.
5 Bake the underside layer of biscuits: Put aside 12 of the biscuit rounds. Place the remaining four biscuit rounds on the underside of a 2-quart glass or ceramic casserole dish. Reduce up the remaining scraps of dough and jigsaw-puzzle the remaining dough into the underside of the casserole dish, ensuring to cowl the underside in a single layer.
Place on a rimmed baking sheet (for ease of elimination from the oven and to catch any drips). Bake within the oven for 15 minutes or till the highest of the biscuits begin to look dry however not brown in any respect. (There is likely to be a slight darkening across the edges, however don’t be alarmed if the biscuits nonetheless look pale white.)
6 In the meantime, make the gravy: Whereas the underside biscuits are baking, make the gravy by inserting the oil, mushrooms, and onions in a big sauté pan. Cook dinner on excessive warmth, stirring continuously, till the mushrooms begin to soften, and the onions begin to look translucent, about 5 minutes. Add the sausage, breaking it up with a wood spoon or spatula, and prepare dinner till the sausage begins to brown, about 5 minutes.
Hold the warmth on excessive, and add the flour, sage, black pepper, nutmeg, and salt to the pan. Stir till the elements are absorbed. Pour within the milk, cut back warmth to medium, and prepare dinner, persevering with to stir till the liquid has began to bubble and thicken, about three to 7 minutes. You’re now able to assemble the casserole!
If the primary layer of biscuits isn’t accomplished baking, decrease the warmth on the gravy to a simmer till the biscuits are accomplished. In case you nonetheless want time to complete the gravy, however the biscuits are accomplished—take away the casserole dish from the oven, and let it sit on the counter or stovetop whilst you end the gravy.
7 Assemble and bake the casserole: As soon as the gravy has thickened and the primary layer of biscuits is completed baking, pour or spoon the gravy over the biscuit layer within the casserole dish. Place the 12 reserved biscuits over the gravy and brush the highest of the biscuits with the melted butter.
Return the dish to the oven and bake till the biscuits are golden brown and the gravy is effervescent and thick, about 18 to 23 minutes.
eight Cool and serve: Let cool 10 minutes earlier than serving heat from the casserole dish.