In a medium saucepan or wok, combine the onion, dashi, sake, soy sauce, and sugar. Bring to a gentle simmer over medium heat.
120 g onion, 120 ml dashi, 60 ml sake, 30 ml light soy sauce, 15 g sugar, salt
Cook, stirring occasionally, until the onion softens, about 5 minutes.
Add the beef and cook over medium-high heat, stirring, until the beef is cooked through and the liquid has reduced by half, about 5 minutes.
250 g beef
Stir in the ginger and simmer for 1 more minute. Season to taste with salt and sugar.
1 teaspoon ginger
Divide the rice among bowls and top with the beef and sauce. Garnish each bowl with a poached egg (if using), sliced green onions, beni-shoga, and togarashi. Serve immediately.
2 servings cooked white rice, 2 poached eggs, green onions, beni-shoga, togarashi
Notes
For the best texture, slice the beef very thinly; freeze it briefly before slicing to make this easier. Otherwise, ask your butcher to slice it for you.