Cook hash brown according to packaging instructions, about 7 minutes. Remove and transfer half of it to the casserole. Also layer half the sliced mushrooms in the casserole. Sprinkle with ¼ teaspoon salt and ¼ teaspoon ground pepper.
Brown sausage in a medium sized fry pan. No oil is necessary. This should take about 5 to 6 minutes. Add onion and green pepper. Continue to cook for another 3 minutes. Transfer to the casserole. Spread it evenly over the hash brown and mushrooms.
Add the remaining mushrooms followed by the hash brown. Sprinkle with ¼ teaspoon salt and ¼ teaspoon ground pepper.
Crack the eggs into a medium sized bowl. Add remaining ¼ teaspoon salt and ¼ teaspoon ground pepper. Pour in milk. Whisk lightly to combine.
Pour egg mixture into the casserole. Spread shredded cheese evenly over the top. Bake in a preheated 350˚F (180˚C) oven for 40 minutes.