Preheat the oven to 400°F (200°C). Scrub the sweet potatoes, prick them all over with a fork, and rub with 1 tablespoon of the olive oil and a generous pinch of salt. Place directly on the oven rack, with a baking sheet on the rack below to catch drips. Bake for 45 to 55 minutes, until a skewer slides through without resistance.

While the potatoes bake, heat the remaining 1 tablespoon olive oil in a skillet over medium heat. Add the diced red onion and cook, stirring, until softened, about 4 minutes. Stir in the garlic, cumin, chili powder, and smoked paprika and cook 30 seconds. Add the black beans, corn, and a pinch of salt. Cook until warmed through, about 5 minutes. Off the heat, stir in the lime juice.

Cut a slit down the center of each baked potato. Push the ends to open it, fluff the flesh with a fork, and season with a tiny pinch of salt. Divide the black bean filling among the potatoes.

Top with cotija cheese, if using, then add avocado slices and cilantro. Serve with lime wedges.