Could anybody advise if I can leave an assembled breakfast casserole in the fridge overnight in a cast iron skillet and bake it the next morning? Would the pan be damaged? Or the food be unsafe to eat? Thanks for your help.
It wouldn't be unsafe to eat, but the overnight storage would be bad for the seasoning on the cast iron. Depending, it could even result in you needing to reseason the pan from scratch.
I suggest instead using a glass or ceramic baking dish.
It wouldn't be unsafe to eat. But depending on the acid content of the food involved (tomato? lemon?) there might be enough iron leaching into it to change the taste. In addition, the seasoning on the pan might suffer from being exposed to much humidity. I'd use a different pan, like a pyrex or ceramic dish.