Lightly grease (use cooking spray) a 7x11-inch baking pan.
Cut the croissants into pieces. With large croissants I like to cut each into 6-8 smaller chunks. Arrange half the croissant pieces in the prepared baking pan.
4 large croissants
Cut the cream cheese into one-inch cubes and evenly arrange over the bread in the pan.
8 ounces cream cheese
Sprinkle the blueberries over the cream cheese, then top with the remaining bread.
1 cup blueberries
Whisk eggs, milk, vanilla, and syrup together in a large bowl until well combined. Pour over the bread in the pan to evenly cover. Cover the entire mixture with foil and refrigerate overnight.
6 eggs, 1 cup milk, ½ teaspoon vanilla, 3 tablespoons maple syrup
The next day, remove the pan from the refrigerator at least 30 minutes before baking. Leave the foil in place. Preheat the oven to 350°F.
Bake the casserole, covered, in the preheated oven for 30 minutes. Uncover and continue baking for an additional 25 to 30 minutes. When done, the center will be firm and the surface lightly browned.
Serve with maple syrup or blueberry sauce.