The quintessential washoku, or traditional Japanese cuisine dish, is nimono — vegetables and other ingredients simmered in a broth of dashi stock, sake, mirin, sugar and soy sauce or miso. Nimono can be made in advance and served warm or cold, saving the cook some effort.
Sometimes the ingredients are sauteed in advance, and other times they are parboiled before being simmered in the broth. A nimono dish can be elegant, but for the most part it's homey fare that is indispensable during winter. The key to great nimono is the quality of the dashi stock, as with many cooked Japanese dishes. Use the best ingredients you can afford for a nice umami taste.
Nishime is a refined type of nimono. Because each ingredient is simmered until there's little liquid left, in the days before refrigeration it kept rather well. Thus a form of nishime became a standard part of osechi ryōri, the food served during the New Year's holiday period, when household cooks had a few days off.