Cut yellow onion into slices. Cut beef slices into 2 if too long.
In a bowl, make a sauce mixture by adding soy sauce, mirin, sake, brown sugar and water. Mix well and set aside.
In a hot frypan, add 1 tbsp of oil and fry onion slices till slightly browned. Add sauce mixture then cover and cook until onions are tender. Add beef slices and fry until beef is no longer pink.
Scoop beef and onions over a bowl of steaming hot rice. Drizzle some sauce and garnish with spring onions. Enjoy.