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 apart with a wooden spoon. Drain excess fat.
03 - Add chopped onion and minced garlic to the skillet. Sauté for 2–3 minutes until onions are translucent and fragrant.
04 - Stir in frozen mixed vegetables and cook for 2 minutes to thaw slightly.
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 all ingredients are well incorporated.
06 - Spoon the mixture evenly into the prepared casserole dish, spreading into a uniform layer.
07 - Cover tightly with aluminum foil. Bake for 35 minutes.
08 - Remove foil and sprinkle remaining 1 cup cheddar cheese over the top. Return to oven and bake uncovered for 10 minutes, or until cheese is melted and bubbly.
09 - Let casserole rest for 5 minutes before serving to allow flavors to settle.