Instructions:
- Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch casserole dish.
- Combine rice and chicken broth in a saucepan. Bring to a boil, then cover, reduce heat, and simmer until liquid is absorbed (about 15–18 minutes). Fluff with a fork and set aside.
- In a large, oven-safe skillet over medium heat, melt 4 Tbsp butter. Add onion and celery, cooking until softened (about 5 minutes). Add garlic and cook for 1 minute until fragrant.
- Sprinkle the flour over the vegetables and cook, stirring constantly, for 1–2 minutes to create a roux.
- Gradually whisk in the milk until smooth. Bring to a gentle simmer, stirring until the sauce thickens slightly.
- Remove skillet from heat. Stir in the cream of chicken soup, thyme, parsley, salt, and pepper.
- Gently fold in the cooked rice, shredded chicken, frozen peas/carrots, and 1 cup of cheddar cheese until everything is evenly coated.
- Pour the mixture into the prepared casserole dish and sprinkle the remaining cheddar evenly over the top.
- In a small bowl, toss the Panko breadcrumbs with 2 Tbsp of melted butter. Sprinkle evenly over the cheese layer.
- Bake covered tightly with foil for 30 minutes. Remove the foil and bake for another 10–15 minutes, or until the topping is golden brown and the filling is bubbling hot throughout.
- Let the casserole rest for 5–10 minutes before serving.