Instructions:
- Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
- Melt 4 Tbsp butter in a large, deep skillet or Dutch oven over medium heat. Add the diced onion and celery; sauté until softened, about 5–7 minutes.
- Sprinkle the flour over the softened vegetables. Whisk constantly for 1 minute to cook out the raw flour taste, creating a roux.
- Gradually whisk in the chicken broth until smooth. Then, slowly whisk in the milk. Bring the mixture to a gentle simmer, whisking frequently, until the sauce thickens enough to coat the back of a spoon (about 5 minutes).
- Remove the sauce from the heat. Stir in the dried thyme, tarragon, garlic powder, salt, and pepper. Taste and adjust seasoning—it should be slightly over-seasoned.
- Gently fold in the shredded chicken, cooked rice, and frozen vegetables into the cream sauce until everything is evenly coated.
- Pour the entire mixture into the prepared baking dish and spread into an even layer.
- In a small bowl, toss the Panko breadcrumbs with the 2 Tbsp of melted butter. Sprinkle the buttered breadcrumbs evenly over the casserole filling, followed by the shredded Cheddar cheese.
- Bake for 30–35 minutes, or until the sauce is bubbling vigorously around the edges and the topping is golden brown and crisp.
- Let the Creamy Chicken and Rice Casserole rest on the counter for 10 minutes before serving to allow the sauce to set.