Step 1: Preheat your oven to 375 °F (190 °C) and lightly grease a 9x13-inch baking dish.
Step 2: In a large mixing bowl, toss the thawed hashbrowns with 3 Tbsp melted butter, the diced yellow onion, diced green bell pepper, ½ tsp garlic powder, and a generous pinch of salt and pepper. Spread this mixture evenly across the bottom of the prepared baking dish.
Step 3: Heat a large skillet over medium heat. Add the 1 lb beef breakfast sausage, breaking it up with a wooden spoon or spatula. Cook for 5–6 minutes, stirring occasionally, until the sausage is browned and fully cooked. Transfer the cooked sausage to a bowl, leaving the drippings in the pan.
Step 4: To the sausage drippings, add 3 Tbsp unsalted butter and melt over medium heat. Sprinkle in 3 Tbsp all-purpose flour and whisk continuously for 1–2 minutes, until the mixture turns a light golden color.
Step 5: Gradually pour 2 cups whole milk into the roux, whisking constantly to prevent lumps. Stir in ½ tsp dried sage, ½ tsp onion powder, and season with salt and plenty of freshly ground black pepper. Continue to simmer, whisking occasionally, until the gravy thickens enough to coat the back of a spoon (about 3–4 minutes).
Step 6: Return the cooked sausage to the thickened gravy and stir until the meat is evenly distributed. Remove the skillet from the heat.
Step 7: Pour the sausage gravy over the prepared hash-brown base in the baking dish, spreading gently to ensure full coverage.
Step 8: Open the tube of refrigerated buttermilk biscuits and arrange them on top of the gravy, spacing them evenly.
Step 9: Place the casserole in the preheated 375 °F (190 °C) oven and bake for 20–25 minutes, until the biscuits are deeply golden and the edges of the gravy bubble around the dish.
Step 10: Remove the casserole from the oven and immediately sprinkle the crumbled turkey bacon (if using) over the hot biscuits. Let the dish rest for 5 minutes before serving.