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 onion and garlic to the skillet; sauté for 2–3 minutes until softened and fragrant.
04 - Stir in frozen mixed vegetables and cook for 2 minutes until heated through.
05 - Remove skillet from heat. Add rice, 1 cup cheddar cheese, sour cream, milk, cream of mushroom soup, beef broth, salt, pepper, paprika, thyme, and cayenne if using. Mix thoroughly until all ingredients are well incorporated.
06 - Spoon the mixture evenly into the prepared casserole dish, spreading smooth.
07 - Cover tightly with aluminum foil and bake for 35 minutes.
08 - Remove foil, sprinkle remaining 1 cup cheddar cheese evenly over the top, and bake uncovered for an additional 10 minutes until cheese is melted and bubbly.
09 - Let casserole rest for 5 minutes before serving to allow it to set.