A few minutes before the sweet potato is ready, begin the sauce. Heat another dash of oil in a wok or large frying pan, and add the spring onions, garlic, chopped chilli, and chopped cashew nuts. Cook over a medium-low heat for a couple of minutes, stirring regularly, until fragrant. Then add the soy sauce, sriracha, honey, peanut butter, and water, and mix well to combine. Allow to cook for another minute or so until it's thick and sticky.