Preheat the oven to 350°F. In a large baking dish, combine the chopped onion, carrots, celery, wild rice, and chicken broth. Cover tightly with foil and bake for 1 hour and 15 minutes.
While the casserole is baking, melt 4 tablespoons of butter in a saucepan over medium heat. Add the flour, salt, and poultry seasoning. Cook for about 1 minute until combined. Gradually whisk in the milk to form a smooth sauce.
If desired, melt the remaining 6 tablespoons of butter in a bowl and toss it with cubed bread for topping.
Once the rice has finished cooking, remove it from the oven. Add in the raw chicken pieces along with the creamy sauce. Stir gently to combine. If using, arrange the buttery bread cubes on top.
Increase the oven temperature to 450°F. Bake uncovered for an additional 20–30 minutes until the chicken is fully cooked and the bread cubes are golden brown.
Garnish with salt, pepper, or freshly chopped parsley before serving.