01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick 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, especially if using frozen berries, to prevent them from sinking during baking.
05 - Sprinkle half of the flour-tossed blueberries over the first layer of croissants. Pour half of the cream cheese mixture over the croissants and berries. Layer with the remaining croissants and blueberries, then pour the remaining cream cheese mixture over the top. Press down gently so the croissant cubes soak up the custard.
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 option.
07 - Bake for 35 to 40 minutes, until the center is set and the top is golden brown.
08 - Let the casserole cool for 10 minutes. Dust with powdered sugar and serve warm with maple syrup or whipped cream, if desired.