01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish generously with butter or nonstick cooking spray.
02 - Distribute the torn croissant pieces evenly across the bottom of the prepared baking dish. Scatter the mixed berries over and between the croissant pieces.
03 - In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, lemon zest if using, and salt until smooth and well combined.
04 - Pour the custard mixture evenly over the croissants and berries. Gently press down on the croissant pieces to help them absorb the liquid.
05 - Sprinkle the turbinado sugar evenly over the surface to create a light, crunchy crust during baking.
06 - Allow the assembled bake to rest at room temperature for 10 minutes so the croissants fully soak up the custard. Extend resting time if the croissants are very fresh.
07 - Bake uncovered for 35 to 40 minutes until the custard is fully set and the top is golden brown.
08 - Remove from the oven and let cool for 10 minutes. Dust with powdered sugar if desired and serve warm.