The day before:Defrost hash browns the night before by placing the bag of frozen hash browns in the refrigerator.
Preheat oven to 375 degrees. Lightly grease a glass pan with butter or oil. Set aside.
In a large skillet, add olive oil and sauté onions until caramelized. Next cook sausage until no longer pink. Drain fat.
Add spinach to the pan and cook until wilted, 2-3 minutes.
Add defrosted hash browns to skillet and cook until lightly brown. Place hash browns in bottom of lightly greased glass pan. Top with sausage and cheese.
Whisk together eggs, salt, pepper, and milk. Pour over sausage and hash brown mixture.