01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a large skillet over medium heat, melt the butter. Add the onion and garlic, sautéing until translucent, about 3 minutes.
03 - In a large bowl, combine the cooked onion and garlic, chicken, uncooked rice, cream of chicken soup, sour cream, chicken broth, peas and carrots, 1 cup cheddar cheese, mozzarella, salt, pepper, paprika, and thyme. Mix until fully combined.
04 - Pour the mixture into the prepared baking dish and spread evenly.
05 - Cover the baking dish tightly with foil. Bake for 30 minutes.
06 - Remove foil, sprinkle remaining ½ cup cheddar cheese evenly over the casserole, and bake uncovered for another 8-10 minutes, or until cheese is melted and casserole is bubbling.
07 - Let stand for 5 minutes before serving. Garnish with fresh parsley if desired.