01 - Preheat the oven to 350°F (175°C). 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 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. Press down gently on the croissant pieces so they begin absorbing the liquid.
05 - Sprinkle the turbinado or granulated sugar evenly over the top for a light crunchy finish.
06 - Allow the assembled bake to rest for 10 minutes so the croissants fully soak up the custard. Extend to 15–20 minutes if the croissants are very fresh.
07 - Bake uncovered for 35–40 minutes until the custard is set and the top is golden brown.
08 - Remove from the oven and cool for 10 minutes. Dust with powdered sugar if desired and serve warm.