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 and no longer pink, breaking it up with a wooden spoon. Drain excess fat.
03 - Add chopped onion and minced garlic to the skillet with the beef. Sauté for 2–3 minutes until onion is softened and fragrant.
04 - Stir in frozen mixed vegetables and cook for 2 minutes until heated through.
05 - Remove skillet from heat. Add uncooked rice, 1 cup shredded cheddar cheese, sour cream, milk, cream of mushroom soup, beef broth, salt, pepper, smoked paprika, dried thyme, and cayenne pepper if using. Mix thoroughly until well combined.
06 - Spoon the mixture evenly into the prepared 9x13-inch casserole dish, spreading it into an even layer.
07 - Cover the dish tightly with aluminum foil. Bake for 35 minutes until rice is tender and mixture is bubbly around edges.
08 - Remove foil and sprinkle remaining 1 cup shredded cheddar cheese evenly over the top. Return to oven and bake uncovered for 10 minutes until cheese is melted and bubbly.
09 - Let casserole rest for 5 minutes before serving to allow it to set and make serving easier.