01 - Preheat oven to 350°F. Grease a 9x13-inch casserole dish with cooking spray or butter.
02 - In a large skillet over medium heat, cook ground beef 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. Sauté for 2–3 minutes until softened and fragrant.
04 - Stir in frozen mixed vegetables. Cook for 2 minutes to thaw and warm through.
05 - Remove skillet from heat. Add uncooked rice, 1 cup cheddar cheese, sour cream, milk, cream of mushroom soup, beef broth, salt, black 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.
08 - Remove foil and sprinkle the remaining 1 cup cheddar cheese evenly over the top. Return to oven and bake uncovered for 10 minutes, or until cheese is bubbly and melted.
09 - Let the casserole rest for 5 minutes before serving to allow the flavors to meld and the texture to set.