Simmer a dashi packet in about 1.5 cups of water for 3–5 minutes. Remove the packet. You'll need 1 cup (200ml) of broth.
Add the dashi broth, soy sauce, mirin, and sugar to a frying pan or shallow pot. Bring to a simmer. Add the sliced onions and cook for about 5 minutes until soft and translucent.
Add the thinly sliced beef, spreading it out so it doesn't clump together. Simmer for 5–7 minutes until the beef is cooked through and has absorbed the flavour.
Scoop hot rice into bowls. Ladle the beef and onions on top, along with some of the cooking liquid. Top with beni shoga and serve immediately.