01 - Preheat oven to 350°F (180°C). Grease a 9x13-inch baking dish generously with butter or non-stick spray.
02 - Scatter half of the cubed croissants evenly across the bottom of the prepared baking dish.
03 - In a medium bowl, beat together the softened cream cheese and granulated sugar until smooth and creamy. Add the eggs, whole milk, vanilla extract, and lemon zest, mixing until fully combined.
04 - Toss the blueberries with the all-purpose flour in a small bowl to coat evenly, which prevents sinking during baking. Sprinkle half of the blueberries over the croissant layer.
05 - Pour half of the cream cheese mixture over the croissants and berries. Layer with the remaining croissants and blueberries, then pour over the remaining cream cheese mixture, pressing down gently to ensure everything is well soaked.
06 - Let the assembled casserole rest for 10 minutes at room temperature to allow the croissants to absorb the custard. Alternatively, cover and refrigerate overnight for a make-ahead preparation.
07 - Bake in the preheated oven for 35-40 minutes, until the custard is set, the top is golden brown, and a knife inserted in the center comes out clean.
08 - Let the casserole cool for 10 minutes. Dust with powdered sugar and serve warm with maple syrup or whipped cream, if desired.