Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch casserole dish with butter or cooking spray.

Cook the wild rice blend according to the package directions. This will typically yield about 3 cups of cooked rice. Set aside.

In a large skillet or Dutch oven, melt 4 tablespoons of butter over medium-high heat. Add the chopped onion and celery and cook, stirring occasionally, until softened, about 5-7 minutes.

Add the sliced mushrooms to the skillet. Cook until they release their liquid and begin to brown, about 6-8 minutes. Stir in the minced garlic and cook for 1 minute more until fragrant.

Sprinkle the all-purpose flour over the vegetables and stir constantly for 1 minute to cook off the raw flour taste.

Gradually whisk in the chicken broth until smooth, followed by the half-and-half. Bring the sauce to a simmer, stirring frequently, and cook until it thickens, about 3-5 minutes. Remove from heat.

Stir the dried thyme, Worcestershire sauce, salt, and pepper into the sauce. Taste and adjust seasoning if necessary.

In a large mixing bowl, combine the cooked wild rice, shredded chicken, and the creamy vegetable sauce. Stir gently until everything is well combined.

Pour the mixture into the prepared casserole dish and spread it into an even layer.

In a small bowl, combine the Panko breadcrumbs, grated Parmesan cheese, and 2 tablespoons of melted butter. Mix until the breadcrumbs are evenly moistened. Sprinkle the topping evenly over the casserole.

Bake, uncovered, for 25-30 minutes, or until the casserole is hot and bubbly and the topping is golden brown.

Remove from the oven and let the casserole rest for at least 10 minutes before serving. This allows it to set up properly.