01 - Preheat oven to 350°F. Lightly grease a 9x13-inch casserole dish.
02 - Steam or blanch broccoli florets for 2 to 3 minutes until just tender but still bright green. Drain and set aside.
03 - In a large skillet over medium heat, melt 2 tablespoons butter. Add onion and sauté for 3 minutes until softened, then add garlic and cook for 1 minute more.
04 - In a large mixing bowl, combine cooked rice, sautéed onion and garlic, blanched broccoli, 1 1/2 cups cheddar cheese, cream of mushroom soup, milk, salt, pepper, paprika, and mustard. Mix until well combined.
05 - Spread the mixture evenly in the prepared casserole dish. Top with remaining 1/2 cup cheddar cheese.
06 - Toss breadcrumbs with melted butter and sprinkle evenly over the casserole.
07 - Bake for 30 to 35 minutes, until the casserole is bubbling and the cheese is melted and golden.
08 - Let rest for 5 minutes before serving.