In a large skillet over medium heat, cook the sausage until browned and crumbly, 8–10 minutes, stirring occasionally. Remove with a slotted spoon and set aside, leaving drippings in the pan.
Reduce heat to medium-low. Melt the butter in the skillet. Stir in the flour and cook for 2 minutes, until lightly toasted.
Gradually whisk in the milk. Increase heat to medium-high and bring to a boil. Reduce heat to medium-low, stir in seasoned salt and black pepper, and simmer 8–10 minutes until thickened. Set aside. (This can be made up to 2 days ahead.)
For the Casserole
Preheat oven to 350°F.
In a medium bowl, whisk together eggs, milk, 1/2 cup cheese, salt, and pepper. Pour into a greased 13x9-inch baking dish.
Reserve 1/2 cup of the cooked sausage. Sprinkle the rest over the eggs. Arrange biscuits evenly over the top.
Drizzle half of the gravy over the biscuits. Sprinkle with the remaining 1 cup cheese and the reserved 1/2 cup sausage.
Bake 30–35 minutes, until the biscuits are cooked through and the top is lightly golden.
Reheat the remaining gravy and drizzle some over the casserole. Serve the rest on the side.