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 the blueberries and lemon zest, if using, over the croissant layer in an even distribution.
03 - In a large mixing bowl, beat the softened cream cheese with an electric mixer until completely smooth. Add the granulated sugar and vanilla extract, blending until fully combined.
04 - Add the eggs one at a time, beating well after each addition until fully incorporated. Pour in the whole milk and heavy cream, whisking until the custard mixture is silky and pourable.
05 - Pour the custard mixture evenly over the croissants and blueberries. Gently press down on the croissant pieces to ensure they begin soaking up the liquid.
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, until the custard is fully set and the top is lightly browned and puffed.
08 - Remove from the oven and let cool for 10 minutes. Dust generously with powdered sugar and serve warm.