Preheat oven to 350°F. Generously grease a 9x13-inch baking dish with cooking spray or butter, ensuring corners and edges are well coated.

Separate the refrigerated biscuits and arrange them flat across the bottom of the prepared dish, pressing down slightly to create an even layer. Small gaps are acceptable.

In a large skillet over medium-high heat, crumble and cook the breakfast sausage, breaking it into small pieces with a wooden spoon. Cook until no pink remains, about 6-8 minutes. Drain excess grease, leaving about 1 tablespoon.

Spread the cooked sausage evenly over the biscuit layer in the baking dish.

In a medium bowl, whisk together eggs, milk, salt, and black pepper until well combined and slightly frothy. Pour this mixture slowly over the sausage layer, allowing it to seep into crevices.

Sprinkle 1 cup of shredded cheddar cheese evenly over the egg layer.

In a small saucepan, prepare the country gravy by whisking the gravy mix with 2 cups of water or milk. Cook over medium heat while whisking constantly until the mixture thickens and bubbles, about 3-4 minutes.

Let the gravy cool for 1-2 minutes, then pour it evenly over the cheese layer. Top with the remaining 1 cup of cheese.

Bake uncovered for 35-40 minutes until the edges are golden brown and the center is set (not jiggly). A knife inserted in the center should come out clean with only gravy or cheese, no raw egg.

Remove from oven and let rest for 10 minutes before cutting into squares. This allows the casserole to firm up for clean slicing.