In a saucepan, heat up the dashi, soy, sake, mirin, and sugar over medium high heat until gently simmering.
Add the onion and cook, until soft and tender, about 2-3 minutes.
Add the thinly sliced beef and simmer until the beef is cooked through and is no longer pink.
Remove from the heat and serve, in a bowl over hot fluffy rice.
Top with green onions and toasted sesame seeds if desired.
