Grease the 9x13 inch baking dish with butter or non-stick spray.
Cut the French bread into 1-inch cubes and place them in the greased baking dish evenly.
In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, sugar, vanilla extract, cinnamon, nutmeg (if using), and salt until fully combined.
Pour the egg mixture evenly over the bread cubes, ensuring all pieces are coated. Gently press the bread down to soak it in the mixture.
Cover the dish tightly with plastic wrap and refrigerate overnight (or for at least 4 hours).
Preheat your oven to 350°F (175°C).
Remove the casserole from the refrigerator and let it sit at room temperature for about 15 minutes.
Bake the casserole in the preheated oven for 45 minutes, or until the top is golden brown and the egg mixture is set.
Once baked, let it cool for a few minutes before slicing.
Serve warm with maple syrup and a sprinkle of powdered sugar, if desired.