Preheat oven to 425°F (220°C). Pierce 6-7 medium sweet potatoes a few times with a fork. Arrange them on the tray and roast for about 40-50 minutes, or until soft. Test either by squeezing or piercing with a fork.
While sweet potatoes are baking, heat the 1 tablespoon vegetable oil in a pan over medium heat. Add 1 ½ cup yellow onion and ½ cup red bell pepper and sauté until softened and translucent onions, about 3-5 minutes. Add 2 cloves garlic and cook for another minute. Sprinkle with salt and pepper to taste. Set aside.
Let the potatoes cool for 5 minutes, or until cool enough to handle. Cut them in half lengthwise (a serrated knife is best) and scoop out the flesh with a spoon, leaving a thin but even layer of potato inside of the skins.
In a large bowl, mix together the sauteed onion mixture, 1 14 oz canned black beans , 1 ½ oz corn kernels, ¼ teaspoon Chili powder, and the scooped out sweet potato flesh. Scoop the filling into the sweet potatoes and top with 1 cup Cotija cheese.
Bake for about 5-10 minutes or until warmed throughout and the cheese starts to crisp.
Serve with additional toppings like Sour Cream, Cilantro, Scallions, 3-4 tablespoon salsa, or jalapenos.