Divide the enoki mushrooms into small bundles.
Place one bundle at one end of each slice of beef and roll tightly.
Layer sliced onion in a pan, then place the beef enoki rolls on top.
In a bowl, mix together the sake, mirin, soy sauce, water, dashi powder (if using), and toasted sesame oil.
Pour the sauce evenly over the beef rolls.
Cover and simmer over medium heat for 15–20 minutes, until the sauce is rich, glossy, and irresistible.
Garnish with sliced scallions and toasted sesame seeds.
Serve immediately over steamed Japanese rice.