Put the sweet potatoes on baking sheet and bake for 30 minutes. Remove from oven, prick a few times with a fork and put back in the oven for 30 more minutes or until tender.
While the sweet potatoes are baking, heat a medium skillet over medium heat. Add the olive oil and the onions. Cook for 2 minutes, until the onions are softened, but not translucent. Add the garlic and cook for 30 more seconds, and then add the cumin, chili powder, and a pinch of salt. Add the tomatoes, beans and corn. Stir until combined; add the cilantro and season with salt and pepper to taste.
To serve, remove the sweet potatoes from the oven, slice down the middle and season lightly with salt. Divide the filling between the potatoes and serve.