Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish. Arrange the cut croissant pieces evenly in the prepared baking dish, ensuring they are well distributed.
Step 2: In a large mixing bowl, vigorously whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, ground cinnamon, nutmeg, orange zest (if you're using it), and a pinch of salt until all ingredients are thoroughly combined and smooth.
Step 3: Carefully pour the egg custard mixture evenly over the croissant pieces in the baking dish. Gently press down on the croissants with a spatula to ensure all pieces are submerged and have a chance to soak up the delicious liquid. For best results, cover and refrigerate for at least 30 minutes, or even overnight, allowing them to fully absorb the custard.
Step 4: In a small bowl, combine the brown sugar, cubed cold butter, and chopped pecans or walnuts (if using). Use your fingers or a pastry blender to mix until the mixture is crumbly. Sprinkle this topping evenly over the surface of the casserole.
Step 5: Cover the baking dish loosely with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and continue baking for an additional 15-20 minutes, or until the casserole is beautifully golden brown on top and set in the center. A knife inserted into the center should come out mostly clean.
Step 6: Allow the casserole to cool for a few minutes before serving. This helps it set further. Dust with powdered sugar if desired, and serve warm with your favorite maple syrup, fresh berries, or a dollop of whipped cream.