FOR THE SAUSAGE GRAVY: Add the sausage to a large skillet over medium heat. Cook, stirring occasionally, until the sausage is browned and crumbly, 8 to 10 minutes. Using a slotted spoon, remove the sausage to a plate; leave drippings in pan.
Add butter and reduce the heat to medium-low to let melt. Sprinkle flour over the sausage drippings, stirring to combine. Let cook for 2 minutes, stirring occasionally, until the flour is lightly toasted.
While stirring, pour in the milk. Increase the heat to medium-high, stirring occasionally. Once the mixture boils and thickens, reduce the heat to medium-low. Add the seasoned salt and black pepper; let simmer for 8 to 10 minutes, stirring occasionally, until thickened. Set aside.
FOR THE CASSEROLE: Preheat oven to 350 F.
Whisk eggs, milk, 1/2 cup cheese, salt and pepper in a medium bowl; pour into a 13-by-9-inch baking dish.
Reserve 1/2 cup of the sausage and sprinkle the remainder over the eggs.
Arrange the biscuits in the baking dish in an even layer. Drizzle half of the gravy over the biscuits; sprinkle with the remaining 1 1/2 cups cheese and the reserved 1/2 cup sausage.
Bake until the biscuits are cooked through and the top is toasted, 30 to 35 minutes.
Reheat the remaining gravy. Drizzle some over the casserole and serve any remaining gravy on the side.
