01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Arrange croissant pieces evenly across the prepared baking dish. Scatter blueberries and lemon zest, if using, uniformly over the croissant layer.
03 - In a large mixing bowl, beat the cream cheese until smooth and creamy. Add granulated sugar and vanilla extract, mixing until fully combined.
04 - Add eggs one at a time, beating well after each addition. Pour in whole milk and heavy cream, whisking until the custard mixture is smooth and pourable.
05 - Pour the cream cheese custard evenly over the croissants and blueberries. Gently press down on the croissant pieces to ensure thorough soaking.
06 - Allow the assembled casserole to rest for 10 to 15 minutes at room temperature so the croissants fully absorb the custard.
07 - Bake uncovered for 35 to 40 minutes, or until the custard is fully set and the top is lightly browned and puffed.
08 - Remove from oven and let cool for 10 minutes. Dust generously with powdered sugar and serve warm.