Lightly grease a 9x13 inch baking dish with butter or cooking spray. Cut the croissants into 1-inch cubes—this size is crucial because it allows the custard to penetrate throughout the bread while maintaining the croissant's tender, flaky structure. Smaller pieces would become soggy; larger pieces won't absorb enough of the delicious custard mixture.

Crack the eggs into a large mixing bowl and whisk together with the heavy cream, vanilla extract, cinnamon, orange zest, salt, and nutmeg until fully combined. Add the maple syrup and Grand Marnier if using, whisking until smooth. I like to let this mixture sit for a minute after whisking so the spices fully bloom and release their flavors into the custard—it makes a noticeable difference in the final taste.

Pour the croissant cubes into the prepared baking dish and distribute them evenly. Slowly pour the custard mixture from Step 2 over the croissants, making sure to coat all the pieces. Gently press down on the croissants with the back of a spoon to help them absorb the liquid, but don't crush them. Cover the baking dish with plastic wrap and refrigerate for at least 8 hours, or up to overnight—this extended soaking allows the croissants to absorb the custard fully while the flavors meld together beautifully.

Remove the baking dish from the refrigerator and let it sit at room temperature for 30 minutes—this is important because it ensures even baking and helps the custard cook through without the edges burning. While the dish is coming to temperature, preheat your oven to 375°F.

Place the baking dish in the preheated 375°F oven and bake for 25-30 minutes, until the top is golden brown and the custard is set but still slightly jiggly in the very center—it will continue to cook slightly as it cools. I find that checking around the 25-minute mark helps you catch it at the perfect point where it's cooked through but still creamy inside.

Let the French toast bake cool for 5 minutes, then dust generously with powdered sugar. Slice into portions and serve warm, topping each piece with fresh berries, whipped cream, and a drizzle of pure maple syrup on the side for extra richness.