Preheat your oven to 350℉. Set a skillet over medium heat and add 1 tablespoon of olive oil, then add your diced onion and cook for 4 to 5 minutes, stirring occasionally, until softened. Then remove from the heat and let them cool slightly.
Combine the remaining ingredients in a mixing bowl, reserving 1 cup of cheese for the topping.
Add the potato mixture to a 9 x 13 baking dish, then top with the remianing cheese.
Bake uncovered for 35-40 minutes until bubbly and the top is lightly browned.
Notes
Make sure to thaw your hash browns before adding them to the casserole.Make Ahead:Prepare the recipe as directed through step 3, but do not bake it. Cover the assembled casserole and refrigerate it for up to 1 day.When you're ready to bake the casserole, remove the dish from the oven and let it sit for 20 to 30 minutes at room temperature to remove the chill. Then bake as directed.To Store:Store leftovers in an airtight container and refrigerate for up to 4 days. To Reheat:Reheat leftovers in a baking dish in a 350°F oven for around 15 minutes until warmed through. Individual portions can be reheated in the microwave.