01 - Preheat oven to 350°F. Grease a 9x13-inch casserole dish with cooking spray or butter.
02 - Heat a large skillet over medium heat. Add ground beef and cook until browned, breaking it up with a wooden spoon. Drain excess fat.
03 - Add chopped onion and minced garlic to the skillet. Sauté for 2-3 minutes until softened and fragrant.
04 - Stir in frozen mixed vegetables. Cook for 2 minutes to thaw and incorporate flavors.
05 - Remove skillet from heat. Add uncooked rice, 1 cup cheddar cheese, sour cream, milk, cream of mushroom soup, beef broth, salt, pepper, smoked paprika, dried thyme, and cayenne if using. Mix thoroughly until well combined.
06 - Spoon the mixture evenly into the prepared casserole dish, spreading it into an even layer.
07 - Cover tightly with aluminum foil. Bake for 35 minutes or until rice is tender and liquid is absorbed.
08 - Remove foil and sprinkle remaining 1 cup cheddar cheese over the top. Bake uncovered for 10 minutes until cheese is melted and bubbly.
09 - Let casserole rest for 5 minutes before serving to allow flavors to set.