In a large skillet, brown the bulk breakfast sausage over medium-high heat, breaking it up with a spatula until no pink remains, approximately 5-7 minutes. Once browned, reduce the heat to medium and add flour, stirring for about 1 minute until the mixture is well combined and bubbly.

Gradually pour in room-temperature whole milk while continuously stirring to prevent lumps. Allow the mixture to cook for 10-12 minutes, stirring often until thickened. Season with seasoning salt, black pepper, paprika, and dry mustard to taste, then remove from heat.

In a mixing bowl, whisk together the eggs and a splash of milk until smooth. Add a pinch of salt and any additional spices, continuing to whisk until fully combined and set aside.

Grease a 9x13-inch baking dish. Cut the refrigerated biscuits into quarters and evenly distribute them across the bottom of the dish. Pour the egg mixture over the biscuits and top with shredded cheddar cheese.

Carefully pour the warm sausage gravy over the assembled casserole. Press down gently if needed.

Preheat the oven to 350°F and bake uncovered for 38-45 minutes until golden brown and bubbly. If prepared overnight, cover with foil and bake for 30 minutes, then uncover and bake for an additional 15-30 minutes.