Slice the onion thinly. In a skillet, combine water, soy sauce, sugar, mirin, sake, grated ginger, and dashi powder, then bring to a boil.
Add the sliced onions to the sauce, cover, and simmer over medium heat for 5 minutes until tender.
Add the thinly sliced beef into the skillet, bring to a boil, and skim off any foam as it comes to a boil.
Reduce the heat to low and simmer for 15 minutes until the flavors are fully absorbed and the beef is tender.
Place warm rice in a bowl, top generously with the cooked beef and onions, and garnish with pickled red ginger if desired.
