Preheat oven: To 300 F degrees.
Prepare bread: Cut the bread into cubes and place them onto a baking sheet. Place the bread into the oven and bake for 10 minutes, just enough to dry the bread out but not toast it. Remove from the oven and set aside.
Prepare sausage: In a large skillet, heat the olive oil over medium heat. Add the onion and sausage and cook until onion has softened and sausage is cooked through. Use a wooden spoon to break up the sausage as it cooks. Finally, season with salt and pepper and dry mustard.
Build casserole: Add half the bread cubes to a 9 x 13 inch casserole dish and top with half of the cooked onions and sausage. Next, add half the chopped bell pepper, chopped green onions, and 1 cup of shredded cheese. Repeat layers with remaining bread, onion, sausage, bell peppers, green onions, and cheese.
Prepare the eggs: In a bowl whisk together the eggs and milk and pour over the casserole evenly. Finally, cover the casserole dish with aluminum foil and put in the fridge overnight.
Bake and serve: The next morning, preheat the oven to 350 °F and remove the foil from the casserole dish. Place into the oven and bake for 50 to 55 minutes. You'll know the casserole is done if you insert a knife in the centre and if it comes clean, it's done. Serve hot!