This recipe was a pleasant surprise. I started the onions and sweet potatoes cooking on a very low flame and turned to working on another dish. When I got back to the sweet potatoes, I discovered they were already cooked through, and needed no sweetening.
It helps to use a sweet rich oil like sesame oil in this dish. If you don't use sesame oil, then try using half oil and half butter for the extra richness.
Add the oil to a saucepan with a very low flame. Put the diced onions on a bottom layer, and put the cubed sweet potatoes in on top. Cover and let it sit and slowly cook for around 20 minutes.
When the sweet potatoes look mostly done, add the remaining ingredients, stir, cover again and let sit for another 10 minutes or so.