Follow these simple steps to bring your casserole to life:

1. Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish and set it aside.
2. In a large skillet, cook the crumbled sausage over medium heat until browned and cooked through. Drain any excess fat.
3. In a large bowl, whisk together the eggs, milk, salt, black pepper, and mustard powder until well combined.
4. Add the cubed bread and cooked sausage into the egg mixture, stirring gently to ensure the bread is thoroughly coated.
5. Layer half of the shredded cheese in the bottom of the prepared baking dish. Pour the egg and bread mixture over the cheese.
6. Top with the remaining cheese, and sprinkle the optional chopped bell peppers for extra color and flavor.
7. Bake in the preheated oven for 45-50 minutes, or until the top is golden and the center is set. A knife inserted into the center should come out clean.
8. Let the casserole cool for a few minutes before garnishing with chopped green onions. Slice and serve.

Growing up, I learned from my grandmother that cooking is as much about intuition as it is about following recipes. So feel free to adjust the seasoning to your taste!