Preheat oven to 350 degrees. Grease a 13x9 rectangular pan. Set aside.
In a large bowl, add 6 eggs, ground cinnamon, vanilla extract, and salt. Whisk until eggs are beaten and mixture is combined. Add apple cider and half and half and whisk gently to combine. Dip one slice of bread and soak in custard for a few seconds. Layer in bottom of the greased pan. Repeat with 5 more slices of bread. Set aside while we make the filling.
Make apple filling by adding butter to a large skillet and turning heat to medium. Once the butter has melted, add apples, cider, brown sugar, spices, and salt. Stir together to combine. Cook for about 8 minutes until apples are slightly soft and liquid is syrupy. Turn off heat, add raisins, and toss together.
Pour cooked apples all over the bottom layer of bread slices soaked in custard. Make sure they are in one even layer. Dip remaining slices of bread in remaining custard and place on top of the apples. Pour leftover custard mixture slowly over casserole, ensuring that it's evenly distributed.
Cover casserole with plastic wrap and refrigerate for at least 3 hours and up to 24 hours to allow the bread to soak up as much bread as possible
Bake casserole in preheated oven until fully-cooked and top of bread turns golden brown, about 50-60 minutes. Remove from oven and let it sit for 10-20 minutes before serving.