Preheat the oven: Preheat your oven to 350°F and grease a 9x13-inch baking dish with cooking spray or butter.
Brown the sausage: In a large skillet over medium heat, cook the sausage, breaking it into small crumbles, until browned and no pink remains, about 8 minutes.
Make the gravy base: Sprinkle the flour over the cooked sausage and stir constantly for 1 minute to cook off the raw flour taste.
Whisk in milk: Slowly pour in 2 cups of milk, whisking constantly, and cook until the gravy thickens enough to coat the back of a spoon, about 5 minutes. Season with black pepper, salt and soy sauce if using.
Layer the biscuits: Scatter the torn biscuit pieces evenly across the bottom of the prepared baking dish.
Pour the gravy over: Spoon the warm sausage gravy evenly over the biscuit pieces, making sure every piece gets some coverage.
Whisk the egg mixture: In a separate bowl, whisk the eggs with the remaining 1 cup of milk until fully combined and slightly frothy.
Pour eggs over casserole: Pour the egg mixture evenly over the biscuits and gravy, letting it soak down between the pieces.
Add the cheese: Sprinkle the shredded cheddar cheese evenly over the top of the casserole.
Bake until set: Bake for 35 to 40 minutes, until the center is set and no longer jiggles, and the top turns golden brown.
Rest before serving: Let the casserole rest for 10 minutes before slicing so the layers hold together on the plate.