In a medium-sized pot, bring 4 cups of water to a boil. Add the wild rice blend, cover, and reduce the heat to a simmer. Cook the rice until it is slightly firm to the bite, approximately 5 minutes less than the time indicated on the package instructions. Drain any excess water and set the rice aside.

Preheat the oven to 350°F (175°C). Grease a large casserole dish to prevent sticking.

In a large skillet, melt the butter over medium heat. Add the chopped celery and diced onion, cooking until softened and translucent, about 3-5 minutes.

Increase the heat to medium-high and add the sliced mushrooms to the skillet. Cook until the mushrooms are tender and have released their moisture, approximately 3 minutes.

Sprinkle the flour evenly over the vegetables, stirring to coat them. Gradually pour in the chicken stock and milk, whisking constantly to avoid lumps. Continue stirring over medium-high heat until the sauce thickens and begins to bubble, approximately 5 minutes.

Remove the skillet from heat and stir in the cooked chicken and shredded cheddar cheese. Season generously with salt and freshly ground black pepper. Taste the mixture and adjust the seasoning if necessary.

Add the cooked wild rice to the skillet, mixing thoroughly to ensure the rice is evenly coated with the sauce. Transfer this mixture to the prepared casserole dish, spreading it out evenly.

In a small bowl, combine the panko breadcrumbs, shredded Parmesan cheese, and melted butter. Toss the mixture until the breadcrumbs are evenly coated. Sprinkle this topping over the casserole.

Place the casserole in the preheated oven and bake for 35-40 minutes, or until the topping is golden brown and the casserole is bubbling around the edges. Allow the dish to cool for a few minutes before serving.