Heat the oven to 400°F and coat a 9-by-13-inch baking dish with cooking spray.
Add the long-grain rice, sweet potato, black beans, tomato sauce, broth, cumin and salt to the dish.
Stir well, level the rice below the liquid and seal tightly with foil.
Bake for 45 minutes, then test a center grain and pierce a sweet potato cube with a knife.
Add 1/4 cup hot broth, reseal and bake for 5 to 10 minutes if either the rice or potato is still firm.
Top with cheddar, bake uncovered for 5 minutes and rest for 10 minutes.