Shred the potatoes, skin and all. Alternatively you can use frozen hashed browns as well if you prefer.
Brown the sausage in a medium pan on the stove on high heat, being careful to watch it so it does not burn. Once browned, remove to a dish. You can drain the sausage if you prefer, I did not drain mine as it let off a very small amount of greese.
Add the hashed browns to the still warm sausage pan. Sauté with the onion powder for five minutes, ten if you’re using frozen hashed browns. You want the frozen variety to be completely thawed and slightly warm.
Transfer the potatoes to the prepared casserole dish.
In a medium bowl, combine the eggs, milk, salt, pepper, nutmeg, and mustard. Whisk until combined, about 30 seconds.
Pour the sausage evenly over the potatoes in the casserole dish. Drizzle the egg mixture evenly over the sausage layer. Add ¾ cup of the cheddar cheese on top, and the full cup of Parmesan cheese. You will reserve the ¼ cup of cheddar for ten minutes before the dish is done baking.
Bake at 350° for 35-45 minutes, or until bubbly and golden on the edges. At the 25-30 minute mark, carefully add the remaining ¼ cup of cheddar to the dish and allow it to melt during the last ten minutes of baking. Remove to a wire rack and cool for five minutes. Serve hot with fresh fruit and yogurt. Enjoy!