Heat oil in a large cast-iron skillet over medium-high heat. Add onions and saute until browned in spots, 3 to 5 minutes. Add sweet potato and cook, stirring, until it starts to brown in spots, 5 to 7 minutes. Stir in garlic, jalapeno, cumin and salt; saute until fragrant, about 30 seconds. Add water and cook, scraping up any browned bits, until liquid is absorbed, 3 to 5 minutes. Stir in corn and black beans and cook until heated through. Stir in cilantro and season with salt and pepper. Serve with lime wedges.