Cook the brown rice: Place the rice in a fine mesh strainer and rinse under cold running water. Set aside. Pour 6 cups water into a medium saucepan or pot that has a tight-fitting lid. Bring to a boil and then stir in the rice. Reduce the heat to a low boil and cook, uncovered, for 30 minutes. Then drain the rice into a colander for 10 seconds, immediately return it to the hot pot, cover with the lid, and let it rest for 10 minutes.
Roast the sweet potatoes: Preheat oven to 425° F. Line a rimmed baking sheet with parchment paper. Peel the sweet potato and chop it into ½-inch cubes. Place on the baking sheet. Drizzle with 1 tablespoon olive oil and sprinkle with salt and pepper. Toss to coat. Bake for 20-25 minutes, until sweet potatoes are soft and lightly browned on the edges.
Reduce oven temperature to 350° F. Lightly grease a 13x9-inch baking dish with cooking spray.
Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the onion and corn and cook, stirring often, for 5 minutes.
Add the ground beef to the pan and cook, crumbling, until browned. Drain off any excess grease. Then stir in the garlic and cook for 30 seconds. Turn off the heat.
Add the enchilada sauce, black beans, rice, roasted sweet potatoes, cilantro and 1 cup of the shredded cheese. Stir to combine.
Transfer the contents of the skillet to the baking dish. Sprinkle on the remaining shredded cheese. Bake, uncovered, for 15-20 minutes, until casserole is hot and bubbly and cheese is melted.
Serve casserole with toppings, as desired, such as chopped cilantro, avocado and sour cream.