Brown the crumbled bulk breakfast sausage in a large skillet over medium-high heat, breaking it into small pieces as it cooks, until fully cooked and no pink remains (about 5-7 minutes). Once cooked, sprinkle the flour over the sausage and stir constantly for 1 minute to cook out the raw flour taste and create a roux base. This flour will thicken the gravy and give it body. Slowly add the room-temperature whole milk one cup at a time, stirring well after each addition to prevent lumps from forming. Continue cooking and stirring for 10-12 minutes until the gravy reaches a thick, pourable consistency that coats the back of a spoon. Season with 3/4 teaspoon seasoning salt, 1.5 teaspoons freshly ground black pepper, 1/4 teaspoon dry mustard, and 1/4 teaspoon paprika. I find that freshly grinding the black pepper right into the gravy makes a noticeable difference in flavor depth compared to pre-ground pepper. Remove from heat and set aside.

In a medium bowl, whisk together 6 eggs, 1/2 cup milk, 1/2 teaspoon salt, 1/4 teaspoon dry mustard, 1/4 teaspoon paprika, and black pepper to taste until well combined and smooth. This mixture will be poured over the biscuits to create a custardy, savory layer between the biscuits and gravy. Set aside until ready to assemble the casserole.

Lightly grease a 9x13-inch baking dish with butter or cooking spray. Open the can of refrigerated biscuits and cut each biscuit into 4 equal pieces, then arrange all the biscuit pieces evenly across the bottom of the prepared dish. Pour the egg custard mixture from Step 2 evenly over the biscuits, making sure to coat them well. Sprinkle the 1 cup of shredded cheddar cheese (I use Cabot sharp cheddar for its bold flavor) evenly over the egg layer. Finally, pour the warm sausage gravy from Step 1 over the cheese, distributing it evenly throughout the casserole. The casserole is now ready to either bake immediately or cover and refrigerate overnight.

If baking immediately: Bake uncovered at 350°F for 38-45 minutes until the casserole is golden brown and set throughout. If the casserole was prepared the night before and refrigerated: Cover with foil and bake at 350°F for 30 minutes, then remove the foil and bake for an additional 15-30 minutes until the top is golden brown and the casserole is heated through. The foil prevents the cheese and biscuits from browning too quickly while the cold casserole comes up to temperature. I like to check it around the 40-minute mark with a toothpick in the center—it should come out clean with no wet egg remaining. Let the casserole rest for 5 minutes before serving for easier portioning.