Start by bringing a saucepan of water to a boil, then add the wild rice blend. Cover the pot and let it simmer for about 25-30 minutes, or until the rice is slightly firm. Once done, drain any excess water and set aside.

Preheat your oven to 350°F (175°C).

In a large skillet, melt 3 tablespoons of unsalted butter over medium-low heat. Add finely chopped celery and onion, cooking them until soft and translucent, about 3 to 5 minutes.

Stir in sliced portabella mushrooms and increase the heat to medium-high. Sauté for about 3 minutes until the mushrooms are softened and golden brown.

Sprinkle in 1/4 cup of all-purpose flour, stirring to combine and create a roux. Gradually pour in 2 cups of chicken stock and 1 cup of whole milk, whisking continuously until thickened, about 5 minutes.

Remove from heat and fold in the cooked chicken, shredded cheddar, and parmesan cheese. Season generously with salt and pepper.

In a greased casserole dish, gently fold the cooked wild rice into the cheesy chicken mixture, ensuring even distribution.

Combine 1 cup of panko bread crumbs with 2 tablespoons of melted butter and 1/4 cup of grated parmesan cheese. Mix well and sprinkle over the casserole.

Bake in the preheated oven for 35 to 40 minutes until the top is golden brown and the dish is bubbling. Allow to cool slightly before serving.