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 smooth and creamy. Add granulated sugar and vanilla extract, mixing until fully 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 mixture is smooth and pourable.
05 - Pour the custard mixture evenly over the croissants and blueberries. Gently press down on the croissant pieces to submerge them and ensure even soaking throughout.
06 - Allow the assembled casserole to rest for 10 to 15 minutes at room temperature, giving the croissants time to absorb the custard.
07 - Bake uncovered for 35 to 40 minutes, or until the custard is fully set 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.