In a large skillet, heat over medium-high heat and add crumbled bulk breakfast sausage. Cook for 5-7 minutes, stirring occasionally, until the sausage is browned and fully cooked.

Once the sausage is golden brown, sprinkle all-purpose flour over it and stir for about 1 minute to incorporate the flour. Gradually pour in room temperature whole milk, stirring continuously to avoid lumps. Cook this mixture on medium heat for 10-12 minutes until it thickens into a creamy gravy. Season with black pepper and seasoning salt to taste.

In a medium bowl, whisk together large eggs, a splash of milk, dry mustard, and paprika until smooth and fully combined.

Lightly grease a 9x13-inch baking dish. Cut refrigerated biscuits into quarters and spread them at the bottom of the dish. Pour the egg mixture over the biscuit pieces, ensuring they are adequately soaked, then sprinkle shredded cheddar cheese over the top, followed by the sausage gravy.

If baking immediately, preheat your oven to 350°F (175°C). Cover the casserole with aluminum foil and bake for 38-45 minutes, until bubbly and the biscuits have puffed up. For an overnight option, cover and refrigerate, then bake straight from the fridge, covering for 30 minutes, then remove the foil and bake for an additional 15-30 minutes until golden brown.

Once baked, let the casserole rest for about 5 minutes before serving. Cut into squares and enjoy!