01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Arrange the croissant pieces evenly across the bottom of the prepared baking dish. Scatter blueberries and lemon zest (if using) uniformly over the croissant layer.
03 - In a large mixing bowl, beat the softened cream cheese until completely smooth. Add granulated sugar and vanilla extract, mixing until well combined.
04 - Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Pour in the whole milk and heavy cream, whisking until the custard is silky and pourable with no lumps remaining.
05 - Pour the cream cheese custard evenly over the croissants and blueberries. Gently press down on the croissant pieces to submerge them slightly and ensure even soaking throughout.
06 - Let the assembled casserole rest at room temperature for 10 to 15 minutes, allowing the croissants to fully absorb the custard mixture.
07 - Bake uncovered for 35 to 40 minutes, or until the custard is fully set in the center and the top is lightly golden brown.
08 - Remove from the oven and let cool for 10 minutes. Dust generously with powdered sugar and serve warm.