Instructions:
- Preheat oven to 375°F (190°C). Lightly grease your 9x13 inch baking dish.
- Cook Rice: Rinse the rice blend. Combine rice and chicken broth in a saucepan. Bring to a boil, reduce heat to low, cover, and simmer until liquid is absorbed (30–40 minutes). Shred chicken if needed.
- Sauté Aromatics: In a large skillet, melt 2 tablespoons of butter over medium heat. Add diced onion and celery. Sauté until soft, about 5–7 minutes.
- Cook Mushrooms: Add the sliced mushrooms and cook until they release moisture and begin to brown slightly.
- Make the Roux: Push vegetables aside. Add the remaining 2 tablespoons of butter to the empty space. Once melted, sprinkle in the flour and whisk constantly for 1 minute to cook out the raw flour taste.
- Deglaze (Optional): Pour in the white wine and scrape up any browned bits from the bottom of the pan. Let it reduce by half.
- Create the Sauce: Slowly whisk in the milk until the sauce is smooth. Continue whisking over medium heat until the sauce thickens enough to coat the back of a spoon.
- Season & Combine: Remove from heat. Stir in the cooked rice, shredded chicken, thyme, parsley, salt, and pepper. Taste and adjust seasonings.
- Transfer: Pour the entire mixture into the prepared 9x13 inch baking dish.
- Prepare Topping: In a small bowl, toss the panko breadcrumbs with the 2 tablespoons of melted butter and the grated Parmesan cheese (if using).
- Top & Bake: Sprinkle the topping evenly over the casserole mixture. Bake for 35–40 minutes, or until the sauce is bubbling and the topping is deep golden brown.
- Rest: Let the casserole rest for 10 minutes before serving to allow the sauce to set.