Step 1: Prepare the Bread and Sausage Base
Generously grease a 9×13 inch baking dish with butter or non-stick cooking spray. Spread the 8 cups of cubed bread evenly in the prepared dish. In a large skillet over medium-high heat, cook the breakfast sausage, breaking it up with a wooden spoon or spatula, until it is thoroughly browned and cooked through. If using onions and bell peppers, add them to the skillet during the last 3-4 minutes of cooking to soften them. Using a slotted spoon, transfer the cooked sausage and vegetable mixture out of the skillet, allowing the excess grease to drain away. Sprinkle this mixture evenly over the bread cubes.

Step 2: Add the Cheesy Layer
Sprinkle the shredded cheese evenly over the sausage and bread layers. Using a combination of cheeses here creates a more complex flavor and perfect melt.

Step 3: Create the Egg Custard
In a large mixing bowl, whisk the 6 eggs vigorously until they are well-beaten and uniform in color. Add the whole milk, heavy cream, dry mustard powder, salt, black pepper, and garlic powder. Whisk everything together until the mixture is completely smooth and homogenous. This custard is the heart of the dish, so ensure it is well-combined.

Step 4: The Crucial Soaking Step
Slowly and carefully pour the egg custard mixture evenly over the entire casserole. Use a fork to gently press down on any bread cubes that are not submerged, ensuring every piece gets a chance to soak up the delicious custard. This is the key to a uniform, non-watery texture.

Step 5: The Resting Period (Overnight or Short-Term)
At this point, cover the baking dish tightly with plastic wrap and refrigerate it for a minimum of 4 hours, but ideally overnight (8-12 hours). This resting period is non-negotiable for the best texture. It allows the bread to fully absorb the custard, resulting in a moist, cohesive casserole that bakes up light and tender, rather than dry and eggy.

Step 6: Bake to Golden-Brown Perfection
When ready to bake, preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and take off the plastic wrap. Let it sit on the counter for 20-30 minutes while the oven preheats to take the chill off (this promotes more even baking). Place the dish in the preheated oven and bake for 45-55 minutes. The casserole is done when the top is puffed and golden brown, the edges are bubbling, and a knife inserted into the center comes out clean.

Step 7: Rest and Serve
Remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the internal structure to set, making it easier to cut into clean squares. Garnish with fresh parsley or chives for a pop of color and freshness