01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick cooking spray.
02 - Scatter half of the cubed croissants evenly across the bottom of the prepared baking dish.
03 - In a medium bowl, beat cream cheese and granulated sugar together until smooth and creamy. Add eggs, whole milk, vanilla extract, and lemon zest, mixing until fully combined and no lumps remain.
04 - Place blueberries in a small bowl and toss with all-purpose flour to coat evenly, which prevents them from sinking during baking. Sprinkle half of the coated blueberries over the croissant layer.
05 - Pour half of the cream cheese custard over the first layer of croissants and blueberries. Arrange the remaining croissant cubes on top, followed by the remaining blueberries. Pour the rest of the custard over everything, pressing down gently so the croissants absorb the liquid.
06 - Allow the assembled casserole to rest for 10 minutes at room temperature so the croissants fully soak up 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 center is set, the top is golden brown, and a knife inserted in the middle comes out clean.
08 - Let the casserole cool for 10 minutes. Dust generously with powdered sugar and serve warm with maple syrup or whipped cream on the side if desired.