Heat a large skillet over medium high heat and add the oil. Add the sweet potatoes and onion and cook, stirring occasionally, about 10 minutes, until the sweet potatoes get lightly browned and start to soften. Add a few tablespoons of water to the skillet, cover with a lid, and steam for another 3-5 minutes until potatoes are cooked through. Uncover and stir in the garlic, chili powder, kale, walnuts, and salt. Cook another 3-4 minutes, stirring often, until kale is wilted. Serve.