Preheat and prepare: Set your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish to prevent sticking and make cleanup easy.
Cook the sausage: In a large skillet over medium-high heat, cook the sausage until browned and crumbly, about 6–8 minutes. Stir occasionally and break the sausage apart as it cooks. Once done, remove from heat and drain excess grease, leaving about a teaspoon in the pan for added flavor.
Make the gravy: Using the same skillet, melt the butter over medium heat. Whisk in the flour to form a roux and cook for 1–2 minutes until lightly golden. Gradually whisk in the milk, stirring constantly until the mixture thickens to a creamy gravy consistency, about 3–4 minutes. Season with salt and pepper to taste, then remove from heat and set aside.
Layer the hashbrowns and sausage: Spread the frozen hashbrowns evenly in the prepared baking dish. Top with the cooked sausage, distributing it evenly for consistent flavor in every bite. Pour the warm gravy over the sausage layer, spreading it to cover the top completely so it seeps through the potatoes.
