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 eggs one at a time, ensuring each is fully incorporated before adding the next. Pour in whole milk and heavy cream, whisking until the custard is smooth and pourable.
05 - Pour the custard mixture evenly over the croissants and blueberries. Gently press down on the croissant pieces to ensure they absorb the liquid. Let the casserole rest for 10 to 15 minutes.
06 - Bake uncovered for 35 to 40 minutes, or until the custard is set in the center and the top is lightly browned and puffed.
07 - Remove from the oven and allow to cool for 10 minutes. Dust generously with powdered sugar before serving warm.