01 - Preheat the oven to 350°F. Grease a 9×13-inch baking dish with butter or nonstick cooking spray.
02 - Arrange 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 fully combined and smooth.
04 - Pour the custard mixture evenly over the croissants and berries. Gently press down on the croissant pieces so they begin absorbing the custard liquid.
05 - Sprinkle the turbinado or granulated sugar evenly over the surface to create a light, crunchy crust during baking.
06 - Allow the assembled dish to rest for 10 minutes so the croissants fully absorb the custard. If the croissants are very fresh, extend the resting time slightly.
07 - Bake uncovered for 35 to 40 minutes, or 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.