Grease the baking dish: Lightly grease a 9x13-inch baking dish with butter or cooking spray. Set it aside while you prepare the sausage mixture.

Brown the sausage: In a large skillet over medium heat, cook the beef sausage with the onion and bell pepper, breaking the sausage apart with a spatula, until no pink remains, about 7 minutes. Drain any excess fat.

Layer the bread: Spread the cubed bread evenly across the bottom of the prepared dish. Scatter the cooked sausage mixture over the bread.

Add the cheese: Sprinkle 1 1/2 cups of the shredded cheddar over the sausage and bread layer, reserving the rest for the top.

Whisk the egg custard: In a large bowl, whisk together the eggs, milk, Dijon mustard, soy sauce mixture, salt and pepper until smooth and pale yellow.

Pour over the layers: Pour the egg mixture evenly over the bread, sausage and cheese. Press down gently with a spatula so every piece of bread soaks up some liquid.

Refrigerate overnight: Cover the dish tightly with plastic wrap or foil and refrigerate for at least 8 hours or overnight, so the bread fully absorbs the custard.

Preheat the oven: The next morning, remove the casserole from the fridge and preheat your oven to 350°F. Let the dish sit on the counter for 15 minutes while the oven heats.

Bake uncovered: Uncover the dish and bake for 45-55 minutes, until the center is set and no longer jiggles and the top turns golden brown.

Add remaining cheese: During the last 10 minutes of baking, sprinkle the reserved cheddar cheese over the top and return it to the oven until melted.

Rest before serving: Remove the casserole from the oven and let it rest for 10 minutes so the slices hold together when cut. Serve warm.