Preheat the oven to 400℉. Mist a 9×13-inch (or other 3-quart) baking dish well with nonstick cooking spray.

Cut the root ends off the green onions, then slice the white and light green parts; set aside. Finely chop the dark green parts and set aside for the biscuit dough.

In a very large skillet (I use this enamel-coated cast iron skillet) over medium-high heat, heat the oil. Once hot, add the mushrooms, sausage, and the white and light green parts of the green onion. Saute over medium heat for 7-10 minutes, breaking up the sausage and any large mushrooms. Do not drain the skillet.

Meanwhile, in a mixing bowl, make the biscuit dough. Combine the 2 cups flour, baking powder, ¾ cup of the cheese, garlic powder, and salt. Stir in the reserved dark green parts of the green onions.

Using a pastry blender or two knives, cut the 6 tablespoons of cold butter into the dry ingredients until the mixture resembles coarse crumbles. Then, add the 1 cup milk and vinegar; stir until the mixture forms a soft slightly sticky dough.

Back at the stovetop skillet, once the sausage is browned, sprinkle the ½ cup flour over top and stir up everything from the bottom of the pan, mixing until the flour is incorporated.

Reduce the heat to medium-low and add the broth, 1 cup at a time, stirring after each addition. Be patient and stir slowly as you continue adding the liquid, bringing it back to a low simmer after each addition so the mixture thickens. After you’ve added all the broth, add the milk in the same manner. After the last addition of milk, return the gravy mixture to a low simmer until thickened.

Stir in the garlic powder, black pepper, sage, and salt. Carefully pour gravy mixture into the prepared casserole dish.

Using two spoons or a cookie dough scoop, drop the dough into eight mounds over the gravy layer; gently spread them out a little bit. It’s OK if the top is not completely covered with the biscuit dough, as the biscuits will expand during baking.

Bake for 15 minutes. Then, remove the casserole dish from the oven and sprinkle the remaining ¼ cup cheese over the biscuits and sprinkle with the flakey salt. Return to the oven and bake for an additional 12-15 minutes.

Allow the casserole to cool at least 10 minutes before serving. The gravy will continue to thicken as it cools a bit. Store leftovers in an airtight container in the fridge for up to 4 days.