Preheat oven to 350, if baking right away.
In a 9x13 casserole dish, combine the hash browns, cooked sausage, onion, peppers and 1 ⅓ cups shredded cheese. Mix well and spread into an even layer.
In a medium bowl, whisk together the eggs, milk, dry mustard, salt and black pepper. Pour over the casserole. Sprinkle the top of the casserole with the remaining ⅔ cup shredded cheese.
Bake at 350 for 1 hour. Or if preparing ahead, cover with plastic wrap and refrigerate for up to 24 hours. Remove from the refrigerator while you preheat the oven and add 5-10 minutes to the baking time to ensure the casserole gets cooked through and hot.
Let rest for 5 minutes to allow it to set up, then slice and serve.
