01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish generously with butter or nonstick 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, distributing them uniformly.
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 so they begin absorbing the liquid.
05 - Sprinkle the turbinado or granulated sugar evenly over the top to create a light, crisp crust during baking.
06 - Allow the assembled bake to rest for 10 minutes so the croissants 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 - Cool for 10 minutes, then dust with powdered sugar if desired. Serve warm.