Preheat your oven to 350°F (175°C). In a large skillet, cook the breakfast sausage over medium heat until it's crumbled and browned. Remove the sausage from the pan and set aside, leaving any drippings in the skillet.

In the same skillet, sauté the chopped bell peppers, onion, and minced garlic until they're softened and fragrant, about 5-7 minutes. Remove from heat.

In a large mixing bowl, whisk together the eggs, whole milk, mustard powder, salt, and pepper until well combined.

In a greased casserole dish, layer the cooked sausage, sautéed vegetables, cubed sourdough bread, and both cheeses evenly.

Pour the egg mixture over the layered ingredients, ensuring everything is well soaked. Cover the dish with foil and refrigerate for at least 1 hour, or up to 16 hours if preparing in advance.

When ready to bake, remove the foil and bake the casserole in the preheated oven for 20 minutes. Then, continue baking for an additional 30-35 minutes or until the top is bubbly and puffed.

Extra Tips:

For a more customized flavor, consider adding in other vegetables such as mushrooms, spinach, or tomatoes. You can also experiment with different types of cheese for varied taste and texture.

If you prefer a spicier kick, try adding diced jalapeños or a dash of hot sauce to the egg mixture. Make sure to let the casserole cool slightly before serving to allow the layers to set. Enjoy your delicious sourdough breakfast casserole!