In a blender, make the sweet and salty marinate by pureeing the onion, scallions, garlic, soy sauce, orange juice, mirin, sesame oil, sugar, and sesame seeds until smooth.
Marinate the meat with one cup for at least thirty minutes. (Save the rest of the marinate for other dishes.) I prefer to cover and refrigerate the meat overnight.
Bring the beef to room temperature before cooking.
Add some cooking oil into a wok and cook the meat on medium-high to high heat. It takes about 3 to 5 mins to thoroughly cook it.