Thinly slice the onion and cut the green onion diagonally into thin slices. Cut the thinly sliced beef into 3-inch pieces.

In a cold frying pan, combine the dashi, sake, mirin, soy sauce, and sugar. Stir to dissolve the sugar.

Separate the onion layers and spread them throughout the cold broth. Distribute the beef slices over the onions in a single layer.

Cover the pan and heat to medium. Once simmering, reduce to low heat and cook covered for 3-4 minutes, skimming off any scum once or twice.

Sprinkle the green onions on top and cook covered for 1 additional minute.

Divide the cooked rice into donburi bowls. Top with the beef and onion mixture, drizzle with the pan sauce, and garnish with pickled red ginger.