01 - Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch (23x33 cm) baking dish with butter or nonstick spray.
02 - In a large skillet over medium heat, melt butter. Add finely chopped onion and minced garlic, and sauté until translucent, about 3 minutes.
03 - In a mixing bowl, combine sautéed onion and garlic with diced chicken, uncooked rice, condensed cream of chicken soup, sour cream, chicken broth, peas and carrots, 1 cup cheddar cheese, mozzarella, salt, pepper, paprika, and thyme. Stir until mixture is well blended.
04 - Spread the chicken and rice mixture evenly into the prepared baking dish.
05 - Cover tightly with foil and bake for 30 minutes.
06 - Remove foil, sprinkle remaining 1/2 cup cheddar cheese evenly over the top, and bake uncovered for an additional 8–10 minutes, until the cheese is fully melted and the casserole is bubbling.
07 - Allow the casserole to stand for 5 minutes before serving. Garnish with fresh parsley if desired.