Sprinkle the cinnamon sugar over the top of the croissants.
Set aside while the oven preheats.
Preheat oven to 350 degrees F.
Bake, covered, for 25 minutes.
Uncover and continue to bake until golden brown, about 10-15 minutes.
Notes
Croissants: The croissants that you'll need are the large sized croissants that you can get at Costco that are about 2.3 ounces each. If you can't get the large croissants, simply use more of the smaller croissants to fill the pan as shown in the "how to make" section above.
Whole milk: You can use half-and-half or cream for richer French toast or use 1% or 2% milk for French toast that isn't as rich. You could also use plant-based milk, like almond milk, if you would like to make the French toast casserole dairy free.