01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Arrange 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 with an electric mixer until completely smooth and free of lumps. Add granulated sugar and vanilla extract, blending until fully incorporated.
04 - Add eggs one at a time, beating well after each addition until smoothly combined. Pour in whole milk and heavy cream, whisking until the custard is silky and pourable with no streaks remaining.
05 - Pour the custard mixture evenly over the croissants and blueberries. Gently press down on the croissant pieces to submerge them and ensure thorough soaking throughout the casserole.
06 - Allow the assembled casserole to rest at room temperature for 10 to 15 minutes, giving the croissants time to fully absorb the custard.
07 - Bake uncovered for 35 to 40 minutes until the custard is firmly set in the center and the top is golden brown.
08 - Remove from the oven and let cool for 10 minutes. Dust generously with powdered sugar and serve warm.