In a saucepan, bring 4 cups of water to a boil. Add the wild rice blend, cover, and simmer for about 45 minutes. Drain excess water and set aside to cool.

Preheat your oven to 350°F (175°C).

Melt 2 tablespoons of unsalted butter in a large skillet over medium-low heat. Add chopped celery and onion; cook for about 5 minutes until soft.

Increase heat to medium and add sliced portabella mushrooms. Sauté for 5-7 minutes until softened.

Sprinkle in ¼ cup of all-purpose flour and stir continuously. Gradually pour in 3 cups of chicken stock and 1 cup of whole milk, whisking until smooth. Cook for about 5 minutes until thickened.

Remove skillet from heat. Stir in shredded chicken and 1 cup of shredded cheddar cheese. Season with salt and pepper.

In a large mixing bowl, combine the cooked wild rice with the chicken-sauce mixture. Fold gently until fully incorporated and transfer to a greased casserole dish.

In a small bowl, combine 1 cup of panko bread crumbs with ½ cup of shredded Parmesan cheese and drizzle with 2 tablespoons of melted butter. Mix well and spread over the casserole.

Bake in the preheated oven for 35-40 minutes until the top is golden brown and bubbly. Let cool slightly before serving.