01 - Preheat oven to 350°F (180°C). Generously grease a 9×13-inch baking dish with butter or nonstick spray.
02 - Scatter half of the cubed croissants evenly across the bottom of the prepared baking dish, creating a uniform base layer.
03 - In a large mixing bowl, beat the softened cream cheese and granulated sugar together using an electric mixer or whisk until completely smooth and creamy. Add the eggs, whole milk, vanilla extract, and lemon zest, then mix until well combined and no lumps remain.
04 - Place the blueberries in a small bowl and toss gently with the all-purpose flour to coat evenly. This prevents the berries from sinking during baking, especially important when using frozen blueberries.
05 - Sprinkle half of the flour-coated blueberries over the first croissant layer. Pour half of the cream cheese custard mixture evenly over the croissants and berries. Arrange the remaining croissant cubes on top, followed by the remaining blueberries. Pour the rest of the custard mixture over everything, pressing down gently with a spatula or your hands to ensure the croissants soak up the liquid.
06 - Allow the assembled casserole to rest at room temperature for 10 minutes so the croissants absorb the custard. Alternatively, cover tightly and refrigerate overnight for a make-ahead option.
07 - Bake in the preheated oven for 35 to 40 minutes, until the top is golden brown and the center is set with only a slight jiggle. A knife inserted near the center should come out mostly clean.
08 - Remove from the oven and let cool for 10 minutes. Dust generously with powdered sugar using a sieve. Serve warm with maple syrup or whipped cream on the side, if desired.