Preheat oven to 350˚F. Grease a large 11 x 15 inch baking dish with 1 teaspoon butter.
Re-hydrate the hash browns as per package directions.
Remove casing from sausages* and scramble fry in a large skillet. Set aside
Saute onions until opaque, add mushrooms and cook until moisture is mostly evaporated. Add hash browns, cooked sausage, and spinach. Stir and fold everything into greased baking dish.
Sprinkle feta and shredded cheese evenly over mixture.
In a large bowl, whisk eggs with cream and season with salt and pepper. Pour over hash brown mixture. Shake pan to ensure even distribution.
Cover with foil and bake for 30 min. Remove foil, reduce heat to 325˚F and bake 15 min longer, or until egg is no longer jiggly in the center.