I'm glad you asked because Nikujaga is best made ahead of time! If you're planning on eating it that day, I recommend making it a few hours in advance and then letting it cool to room temperature once. The changes in temperature and time soaking in the broth helps all the ingredients release and reabsorb flavors from the other ingredients in the stew. The only thing you'll want to do is hold off on adding anything green until you reheat your Nikujaga; otherwise, it will get overcooked and lose its color.