Preparation of Wild Rice: In a medium-sized pot, bring approximately four cups of water to a vigorous boil. Add the wild rice blend, cover, and reduce the heat to a simmer. Continue to cook until the rice is slightly firm to the bite, approximately five minutes less than the time suggested on the package instructions.
Oven Preheating: Set the oven temperature to 350 degrees Fahrenheit (approximately 175 degrees Celsius).
Vegetable Sauté: In a large non-stick skillet, melt the butter over medium-low heat. Add the chopped celery and diced onion, cooking until they are soft and translucent, about 3-5 minutes.
Mushroom Cooking: Increase the heat to medium-high and introduce the sliced mushrooms to the skillet. Continue to cook until the mushrooms have softened and released their moisture, roughly 3 minutes.
Roux Creation: Over the vegetables, evenly distribute the flour, stirring to coat. Gradually pour in the chicken stock and milk, stirring continuously. Maintain medium-high heat until the mixture thickens and begins to bubble, which should take about 5 minutes.
Combining Ingredients: To the thickened sauce, add the chopped chicken and shredded cheeses. Season the mixture generously with salt and pepper to taste, then remove the skillet from heat.
Assembling the Casserole: Once the rice is cooked, drain any excess water and immediately stir it into the prepared chicken and sauce mixture. Transfer this mixture into a greased large casserole dish.
Breadcrumb Topping: In a small bowl, combine the panko bread crumbs and shredded Parmesan cheese. Drizzle with melted butter and toss to coat evenly. Sprinkle this breadcrumb mixture over the surface of the casserole.
Baking: Place the casserole in the preheated oven and bake for approximately 35-40 minutes, or until the top is golden brown and the contents are bubbling.