Preheat your oven to 350°F (175°C).

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

Melt butter in a saucepan over medium heat. Whisk in flour, salt, and poultry seasoning for 1 minute. Gradually add milk, whisking continuously until thickened, then remove from heat.

Uncover the dish, stir in raw chicken pieces and creamy sauce, mixing thoroughly. Optionally toss cubed bread with remaining butter and spread over the top.

Increase oven temperature to 450°F (230°C). Bake uncovered for an additional 20 to 30 minutes until chicken is fully cooked and the top is golden brown.

Let casserole sit before serving. Garnish with cracked pepper or parsley if desired, and serve warm.