Preheat your oven to 350°F (175°C).

Cook the wild rice according to package directions, usually about 45 minutes. Drain and set aside.

Melt 2 tablespoons of butter in a large skillet over medium heat. Sauté diced onion, minced garlic, sliced carrots, chopped celery, and sliced mushrooms for about 5-7 minutes until softened.

In a large mixing bowl, whisk together the cooked wild rice, sautéed vegetables, and diced cooked chicken until well combined.

Grease a 9x13-inch baking dish and transfer the mixture into it, spreading evenly.

Combine panko breadcrumbs, melted butter, and grated Parmesan cheese in a small bowl until crumbly. Sprinkle evenly over the casserole.

Bake uncovered for 30-35 minutes until the top is golden brown and edges are bubbling.

Let the casserole rest for about 5 minutes before serving.