Preheat your oven to 350°F (175°C).

In a large baking dish, combine diced onion, sliced carrots, chopped celery, wild rice, and chicken broth. Stir well, cover with foil, and bake for 1 hour and 15 minutes.

In a saucepan, melt 4 tablespoons of butter over medium heat. Whisk in flour, salt, and poultry seasoning, cooking for about 1 minute. Gradually add milk and whisk until smooth and slightly thickened.

If using the topping, melt the remaining 6 tablespoons of butter, toss cubed bread in the butter until coated.

Remove foil from the rice mixture, stir in raw chicken pieces and creamy sauce until combined. Sprinkle buttered bread cubes on top.

Raise the oven temperature to 450°F (230°C) and bake uncovered for 20 to 30 minutes until chicken is cooked through and bread cubes are golden brown.

Let the casserole rest for a few minutes before serving. Garnish with salt, pepper, or parsley as desired.