Nikkorogashi is a simmered Japanese dish that usually consists of taro potato (It's not really a potato, it's a distant relative of the philodendron) and onion simmered in dashi stock and soy sauce. Some variations have mirin added, but when using sweet potato I find that this recipe is sweet enough to not need the added sugar. I used white garnet yams in the photo below, but you can easily swap them out for regular sweet potatoes or even your Idaho baking potato and still get wonderful results.
There are several kinds of dashi, but the most common one now days is in the form of Dashi-no-motto. Dashi-no-motto is a powdered seasoning that can be bough in packets and added to your dishes mixed in a little water to form a broth. Though Dashi-no-motto tastes great, it is made from yeast and loaded with preservatives and msg (unless you are lucky enough to find the msg-less variety). I prefer making my Dashi from scratch and I usually make kombudashi. If you don't have the time to make kombudashi from scratch, the easiest thing to do is just use water. You won't get the depth of flavor that you would have with dashi, but between the caramelized onions and the tamari, you still will have a very tasty side dish.
It is also very important that the pan you use has a lid. The majority of simmered dishes in Japan are simmered under cover and it does affect the flavor of the dish. This belief dates back to times when the Japanese believed that if you left a dish uncovered while cooking that evil spirits would get into your food and spoil the dish. I don't believe in the evil spirits, but your liquid will evaporate before the potatoes are cooked and you'll end up with dried out and flavorless potatoes instead.
1 large sweet potato or yam, diced
1 medium sized onion cut into thin slices
1 Tablespoon/15 ml sesame oil (not toasted!)
1 cup/200ml kombudashi (see link for details)
3 Tablespoons/45 ml Wheat Free Tamari
Heat up a dutch oven or large sauce pot over medium heat and add the oil. Once the oil is smoking hot, add the onions and stir, being careful not to burn them. Once the onions are translucent, add the potato and saute until coated with oil.
Add the kombudashi and tamari. Stir to mix and cover. Turn the heat down to medium low and cook for 10-15 minutes or until the potatoes are tender. Serve warm. This also makes great leftovers as it tastes even better the next day.