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 in 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 well combined and smooth.
04 - Pour the custard mixture evenly over the croissants and berries, pressing down gently so the croissant pieces soak up the liquid.
05 - Sprinkle the turbinado or granulated sugar evenly over the top for a light crunchy finish.
06 - Let the assembled bake rest for 10 minutes to allow the croissants to fully absorb the custard. Extend resting time if the croissants are very fresh.
07 - Bake uncovered for 35 to 40 minutes, or until the custard is fully set and the top is golden brown.
08 - Allow the bake to cool for 10 minutes. Dust with powdered sugar if desired and serve warm.