Preheat the oven to 350°F (175°C).
In a large skillet over medium heat, cook the ground turkey sausage until fully browned and cooked through, breaking it apart with a spatula as it cooks, about 7-8 minutes.
Add the diced onion to the skillet with the sausage and cook until the onion is softened and translucent, about 4-5 minutes. Remove from heat and set aside.
In a large mixing bowl, whisk together the eggs, whole milk, salt, black pepper, garlic powder, and smoked paprika until well combined.
Lightly grease a 9×13 inch baking dish.
Spread the thawed hashbrowns evenly in the bottom of the prepared baking dish.
Evenly distribute the cooked sausage and onion mixture over the hashbrowns.
Sprinkle 1 cup of shredded cheddar cheese evenly over the sausage layer.
Pour the egg mixture evenly over all the layers in the baking dish.
Sprinkle the remaining 1 cup of shredded cheddar cheese evenly on top.
Cover the baking dish tightly with aluminum foil if freezing, or leave uncovered if baking immediately.
To freeze: Place the covered casserole in the freezer for up to 3 months.
To bake from frozen: Preheat oven to 350°F (175°C). Remove foil and bake uncovered for 60-70 minutes, or until the eggs are set and the top is golden brown.
To bake fresh (not frozen): Bake uncovered at 350°F (175°C) for 40-45 minutes, or until the eggs are set and the top is golden brown.
Let the casserole rest for 5 minutes before slicing and serving.