Grease a 9x13 casserole dish.
Cube the crusty bread, and place in casserole dish. Sprinkle the blueberries on top of the bread. Set aside.
In a large mixing bowl, whisk the eggs, half and half, milk, vanilla, brown sugar, cinnamon, and nutmeg. Pour evenly over the the bread and blueberries.
If making this casserole the day before, cover the casserole dish tightly with Saran Wrap. Place in refrigerate overnight.
In a medium bowl, mix together the flour, brown sugar, cinnamon and salt. Add the cubed butter and use a pastry cutter (or your fingers) to combine, pressing until the mixture is crumbled.
If making this casserole the day before, place mixture in a resealable plastic bag in your refrigerator overnight. (Continue with the following step in the morning.)
Pour topping mixture evenly over the casserole right before baking. Bake at 350 degrees for approximately 35-40 minutes.
Serve French toast casserole with a dusting of powdered sugar, a pat of butter, and some warm syrup. Enjoy!