Prepare the Sausage Gravy:
Cook the Sausage: In a large skillet over medium heat, cook the 1 lb ground breakfast sausage until it is browned and crumbled. Remove the cooked sausage from the skillet and set it aside, leaving about 2 tablespoons of drippings in the pan.
Make the Roux: Add the ¼ cup unsalted butter to the skillet with the sausage drippings and melt over medium heat. Stir in the ¼ cup all-purpose flour to create a roux, cooking for 1–2 minutes, stirring constantly, until it’s lightly golden.
Add Milk and Seasoning: Gradually whisk in the 3 cups whole milk (or half-and-half), stirring constantly to avoid any lumps. Continue cooking, whisking frequently, until the mixture thickens to a gravy consistency (about 5–7 minutes). Stir in the ½ tsp garlic powder, ½ tsp onion powder, salt, and black pepper to taste.
Combine with Sausage: Return the cooked sausage to the skillet with the gravy and mix well to combine. Set the finished sausage gravy aside.
Assemble the Casserole:
