Preheat your oven to 350°F (175°C).

In a large skillet, heat olive oil over medium heat. Add the ground beef sausage and cook for 5 to 7 minutes until browned. Drain excess grease.

In a large mixing bowl, combine thawed hashbrowns, cooked sausage, diced onion, garlic powder, salt and black pepper. Mix well.

Whisk together sour cream, cream of mushroom soup, and milk in a separate bowl until smooth.

Pour the creamy sauce over the hashbrown mixture and stir gently until evenly coated.

Grease a 9x13 inch baking dish, spread the hashbrown mixture evenly, and sprinkle cheddar and mozzarella cheeses on top. Bake for 45 to 50 minutes.

Allow the casserole to cool for about 10 minutes before serving.