Preheat the oven to 350°F (175°C).

In a large mixing bowl, combine diced onion, carrots, celery, wild rice, and chicken broth. Transfer to a casserole dish and bake for 1 hour and 15 minutes.

Melt 4 tablespoons of butter in a saucepan, whisk in flour, salt, and poultry seasoning. Gradually add milk, whisking continuously until the sauce thickens, about 5 minutes.

Melt the remaining butter and toss cubed bread until coated. Set aside.

Once the wild rice mixture is cooked, stir in the raw chicken pieces and sauce. If using the topping, sprinkle the buttered bread cubes on top.

Increase oven temperature to 450°F (232°C) and bake uncovered for 20 to 30 minutes, until chicken is cooked and topping is golden.

Remove from oven and let it rest for a few minutes before serving. Garnish with salt, pepper, or fresh parsley if desired.