Coat a 9x13" casserole pan with non-stick spray and set aside.
In a large skillet brown the sausage over medium-high heat, while crumbling into small pieces. Cook until no longer pink, drain any excess fat.
Meanwhile combine the eggs, milk, mustard and pepper and whisk until fully combined. Add in the hash brown potatoes, 1 cup of the shredded cheese and the browned sausage. Stir to combine and pour into the prepared casserole dish.
Cover with foil and refrigerate for up to 12 hours.
When ready to bake, remove casserole from the fridge and preheat oven to 350 degrees F. Cover the top of the casserole with the remaining ½ cup of shredded cheese and bake for 45 to 50 minutes, or until the center is set.
Let rest for 10 minutes before slicing. Top with finely diced green onions and enjoy with your favorite hot sauce!
Notes
O'Brien hash brown potatoes come with peppers and onions already mixed in. You can either purchase this blend from the freezer section or some grocery stores carry them in the refrigerated section (near the eggs).