Add the thawed hash browns to a 9"x13" casserole dish. Spread them out evenly.
OPTIONAL STEP: Melt 4 Tbsp of butter and toss the hash browns in it. Then bake them at 400° F for 30 minutes. Continue with the recipe, most of this can be done while the hash browns are in the oven.
Cook the sausage and onion in a skillet until the meat is browned and the onion is tender. Drain any excess fat.
In a bowl, whisk together the eggs, milk, garlic powder, onion powder, salt, and pepper.
Spread the sausage mixture evenly over the hash browns.
Sprinkle half of the cheese over the top.
Pour the egg mixture over. Cover with foil and bake at 350° F for 1 hour.
Remove the foil and sprinkle on the remaining cheese. Bake for 10 more minutes, or until the cheese melts and starts to brown.
Remove from oven and let stand for 5 minutes before serving.