01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Arrange the torn croissant pieces evenly across the prepared baking dish. Scatter the mixed berries over and between the croissant pieces.
03 - In a large 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 so they begin absorbing the custard.
05 - Sprinkle the top evenly with turbinado or granulated sugar. Let the assembled bake rest for 10 minutes to allow the croissants to fully soak up the custard (extend resting time if the croissants are very fresh).
06 - Bake uncovered for 35 to 40 minutes, or until the custard is fully set and the top is golden brown.
07 - Allow the bake to cool for 10 minutes. Dust generously with powdered sugar and serve warm.