In a sauté pan, cook onions and brown and crumble sausage. Drain any excess grease. (If you’d like, at this point, you can just take crumbled sausage out of pan, leaving the grease, and then brown the hashbrowns a bit, this adds nice flavor to the casserole)
In a medium bowl, whisk together eggs, milk and 1 cup of shredded cheese.
In a baking dish, spread thawed hashbrowns into bottom of dish.
Make sure your hashbrowns are thawed and dried. Pat them dry with a paper towel. You don't want that extra moisture to make your casserole runny.
*For an added layer of flavor, try browning the hashbrowns in a pan before putting them in the baking dish.
Spread cooked sausage on top of hashbrowns. Next, pour egg mixture on top of that. Sprinkle the remaining cup of cheddar cheese on top.
Cover with aluminum foil and bake for about 30 minutes. Take foil off and bake for additional 10 minutes. (Just keep an eye on it to make sure the cheese doesn’t brown up too much).