Grease the 9x13 inch baking dish with butter or cooking spray.
In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, ground cinnamon, nutmeg, and salt until well combined.
Add the cubed bread to the egg mixture and gently fold until all pieces are soaked. Let it sit for a few minutes to absorb the liquid.
Pour the bread mixture into the greased baking dish and spread it out evenly.
Cover the baking dish with plastic wrap or aluminum foil and refrigerate overnight (or for at least 4 hours).
Preheat the oven to 350°F (175°C).
Remove the casserole from the refrigerator and let it sit at room temperature for about 15 minutes.
Bake uncovered in the preheated oven for 45 minutes, or until the top is golden brown and the center is set.
Let the casserole cool for a few minutes before slicing. Serve with maple syrup and a dusting of powdered sugar, if desired.