Preheat the oven:
Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish with cooking spray or butter.
Cook the sausage:
In a large skillet, cook the sausage over medium heat until browned and fully cooked, about 7-9 minutes. Use a spatula to break the sausage into crumbles as it cooks. Drain excess fat if necessary and set aside.
Mix the hashbrowns and cheese:
In a large mixing bowl, combine the thawed hashbrowns, cooked sausage, diced onion, 1 1/2 cups of the cheddar cheese, and 1 cup of the mozzarella cheese.
Prepare the sauce:
In a separate bowl, whisk together the sour cream, cream of mushroom or chicken soup, milk, garlic powder, onion powder, paprika, salt, and pepper. Stir until smooth.
Combine and assemble:
Pour the sauce over the hashbrown and sausage mixture. Stir everything together until evenly coated.
Transfer the mixture to the prepared baking dish, spreading it out evenly.
Sprinkle the remaining cheddar and mozzarella cheeses over the top.
Bake the casserole:
Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 15 minutes, or until the cheese is melted, bubbly, and golden brown on top.
Garnish and serve:
Remove from the oven and let the casserole cool for 5-10 minutes. Garnish with fresh parsley or chives if desired before serving.