Cut the meat in strips Dry the meat and cut into very thin strips against the grain.
Cut the spring onions Cut half a spring onion (green part) into fine diagonal rings and set aside for garnish. Cut the rest of the spring onions into fine round rings.
Chop onion and garlic Peel and finely chop the garlic. Halve the onion. Cut one half into fine cubes, cut the other half into strips and set aside.
Prepare beef marinade Place the sugar, rice wine/mirin, soy sauce, and sesame seeds in a large bowl and stir to dissolve the sugar. Add the spring onions (fine round rings), the garlic and the diced onions and stir everything together.
Add the marinade Add the marinade to the meat and mix well with your hands so that each piece of meat is well marinated.
Refrigerate the meat Place the meat in the fridge for two hours to marinate.