01 - Preheat the oven to 350°F. Grease a 9×13-inch baking dish with butter or nonstick spray.
02 - Scatter the croissant pieces evenly across the prepared dish. Sprinkle the blueberries uniformly over the croissant layer.
03 - In a mixing bowl, beat the softened cream cheese and granulated sugar together until smooth and creamy.
04 - Add the eggs, whole milk, heavy cream, vanilla extract, and lemon zest if using. Whisk until fully combined and the mixture is silky smooth.
05 - Pour the cream cheese custard evenly over the croissant and blueberry layers. Gently press the croissants down with a spatula so they absorb the custard.
06 - Sprinkle turbinado or granulated sugar over the top for a golden, crunchy finish.
07 - Bake uncovered for 40 to 45 minutes, until the center is firmly set and the top is golden brown.
08 - Allow the casserole to rest for 10 to 15 minutes before serving. Dust with powdered sugar if desired. Serve warm or at room temperature.