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