Preheat your oven to 375°F (190°C).

In a medium saucepan, combine the brown rice and vegetable broth. Bring to a boil, then reduce heat, cover, and simmer for about 45 minutes until tender.

Heat olive oil in a large skillet over medium heat. Sauté the diced onion for about 5 minutes until translucent.

Add minced garlic, cumin, chili powder, smoked paprika, and season with salt and pepper. Cook for 1-2 minutes until fragrant.

Incorporate the diced sweet potato into the skillet and cook for about 5-7 minutes until softened.

Stir in black beans, kidney beans, corn, diced bell pepper, and diced tomatoes. Mix well.

Once the rice is cooked, add it to the bean and vegetable mixture, stirring gently to combine.

Transfer the mixture to a greased baking dish, spreading it evenly.

Cover the dish with foil and bake for 25 minutes. Then, remove the foil to brown the top.

Bake uncovered for an additional 15-20 minutes until the sweet potatoes are tender and the top is golden.

Let the casserole cool for a few minutes before serving. Garnish with cilantro, avocado, and lime wedges if desired.