Spread the cubed challah across the bottom of a 9×13 baking dish.
Sprinkle the shredded cheese evenly over the bread cubes.
In a large bowl, whisk together eggs, half-and-half, mustard powder, salt, pepper, garlic powder and onion powder until well combined.
Pour the egg mixture evenly over the bread and cheese in the baking dish.
Cover the dish with plastic wrap or foil and refrigerate it overnight (or at least a couple of hours) to allow the bread to absorb the egg mixture.
Preheat the oven to 350°F.
Remove the casserole from the refrigerator and let it sit at room temperature for about 15 minutes while the oven preheats.
Bake, uncovered, for 45-50 minutes or until the top is golden brown and the casserole is set in the middle. You can insert a knife in the center to check if it’s fully cooked (it should come out clean).
Let the casserole cool for a few minutes before cutting and serving.
