Prepare the Baking Dish:
Grease your 9x13-inch baking dish with unsalted butter to prevent sticking.
Arrange the Bread:
Cut the French bread into thick slices and layer them in the prepared baking dish. You may slightly overlap the bread slices.
Make the Custard:
In a large bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, brown sugar, vanilla extract, cinnamon, nutmeg, and orange zest until well combined.
Soak the Bread:
Pour the custard mixture evenly over the bread slices. Ensure all slices are soaked well with the custard.
Cover and Chill:
Cover the baking dish with plastic wrap and refrigerate overnight, allowing the bread to absorb the custard.
Bake:
Preheat the oven to 350°F (175°C). Remove the plastic wrap and bake the French toast for 40-45 minutes, until golden and set.
Serve:
Once baked, you can drizzle with maple syrup, powdered sugar, or fresh whipped cream. For an added touch, broil for 1-2 minutes to create a crispy caramelized topping.