1-Prep: Dry the Bread and Ready the Custard Cube the 12 slices of thick sliced bread until you have about 14 cups of cubes. To keep the casserole from getting too soggy, dry the bread either overnight on a baking sheet covered lightly with a dish towel or in a hot oven. For oven-drying, preheat the oven to 300°F and bake the bread cubes for 10 minutes. Watch carefully to keep the cubes from browning; the goal is to remove excess moisture, not toast the bread.

In a large bowl, whisk the custard ingredients together: 6 large eggs, 3 cups milk, 1/2 teaspoon dry mustard powder, 1/2 teaspoon salt, and 1/2 teaspoon black pepper. Whisk until combined and slightly frothy. This custard mixture will soak into the bread and set into a creamy center when baked.

2-Assemble: Layering for Even Flavor

1. Grease a 9×13 inch baking dish to prevent sticking.
2. Place half of the cubed bread evenly in the bottom of the dish.
3. Scatter half of the cooked and crumbled bacon (or half of the 1 cup diced ham if using ham) over the bread.
4. Add half of the 3 thinly sliced green onions and half of the 1/2 cup finely diced red bell pepper.
5. Sprinkle half of the 4 cups shredded cheddar cheese over the layers.
6. Repeat the layers with the remaining bread, bacon or ham, green onions, red peppers, and cheese.
7. Pour the egg and milk mixture evenly over the assembled casserole. Press down gently so the bread absorbs the custard.

3-Chill: Overnight or Short Rest Cover the dish tightly with foil or plastic wrap and refrigerate for at least 3 hours or for best results, refrigerate overnight. Chilling allows the custard to soak into the bread, melding flavors and improving texture.

4-Bake: Timing and Finishing Remove the casserole from the refrigerator and let it sit at room temperature while you preheat the oven to 350°F. This helps promote even baking. Cover the baking dish with foil and bake for 45 to 55 minutes. If you are baking multiple pans at once, baking time will likely be on the longer end. To test for doneness, insert a knife into the center; it should come out clean when the casserole is fully cooked.

For a beautifully browned cheese top, remove the foil and broil for about 3 minutes at the end of baking until the top is golden and slightly crispy. Watch closely while broiling to avoid burning.

5-Serving and Leftovers Let the casserole rest for 5 to 10 minutes after baking to make slicing easier. Serve warm, alone or with sides like fruit salad, a green salad, or light vinaigrette.