Thinly slice ½ yellow onion and 1 scallion. Cut 225g of very thinly sliced beef into bite-sized strips.
In a cold skillet, stir ½ cup of dashi, 2 tablespoons of sake, 2 tablespoons of mirin, 3 tablespoons of soy sauce, and 1 tablespoon of sugar until dissolved.
Spread the onion in the broth. Separate the beef slices and spread them over the onion. Cover and place over medium heat.
Once simmering, reduce the heat and cook covered for 3-4 minutes, until the beef is cooked through and the onion is tender. Skim off any foam.
Scatter over the scallion, cover and cook for 1 minute. Serve over 500g of hot cooked Japanese short-grain rice, spooning over some of the broth.
