Peel and grate ginger root. Mix grated ginger, Soy Sauce, Sake, and Mirin in a bowl. Set aside.
Slice brown onion thinly. Cut beef in 2" width pieces.
Heat a frying pan with oil at medium high heat, and cook beef for a couple of minutes until slightly browned. Add onion and stir-fry for a minute. Take care not to overcook. Add the ginger sauce, and cook to coat the meat with the sauce.