If making immediately, preheat the oven to 350 degrees F.
Grease a 9x13" baking dish with cooking spray.
Add the croissants to the baking dish. They should fill the baking dish but not overflow.
Sprinkle the blueberries on top.
In a large bowl, whisk together the eggs, half and half, milk, sugars, vanilla, cinnamon, and salt.
Pour the mixture evenly over the croissants and blueberries, pressing the croissants down to soak up the mixture.
If making ahead of time, cover with plastic wrap or foil and refrigerate.
Prior to baking remove the casserole from the refrigerator and let it sit for 30 minutes.
Preheat the oven to 350 degrees F.
Bake in the preheated oven for 40-50 minutes, until the egg mixture is set and the top is golden brown.
Serve topped with powdered sugar or maple syrup.
Enjoy!