Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or cooking spray.

In a large skillet over medium heat, crumble and brown the breakfast sausage until no pink remains, about 6–8 minutes.

Add the chopped onion in the last 3 minutes and cook until softened. Drain excess fat if needed. Season lightly with salt and lots of black pepper.

Meanwhile, prepare sausage gravy: In a medium saucepan, melt 4 tbsp butter over medium heat. Whisk in 1/4 cup flour and cook for 1–2 minutes until bubbly.

Gradually whisk in 3 cups milk and simmer, whisking frequently, until thickened, about 6–8 minutes. Season with salt and lots of black pepper. Remove from heat.

Spread the thawed hashbrowns in an even layer in the prepared baking dish.

Scatter the cooked sausage and onions evenly over the hashbrowns.

Cut the biscuit dough into bite-sized pieces and scatter over the sausage and hashbrowns.

Pour the sausage gravy evenly over the top, ensuring most biscuit pieces are covered. Sprinkle shredded cheese evenly over everything, and dot with butter if desired.

Bake for 25–30 minutes, until biscuits are golden, cheese is bubbly, and edges are slightly browned.

If the top is browning too quickly, tent with foil for the last 5–10 minutes.

Let the casserole rest for 5–10 minutes before serving. Garnish with chopped chives or green onions.