How long to cook multiple food items with varying temperatures?
The turkey is far more sensitive; you don't want to overcook it.  Casseroles in general just need to be heated through, and perhaps crisped up.

Therefore, I would set the oven to temperature for the turkey, and put the casserole in with it.

If it is not brown enough when the turkey is done, use the broiler to crisp up the casserole.