Cut the beef into cube-shaped pieces.
Slice the onion and green onion.
In a pan, heat cooking oil and stir-fry the beef.
When the meat is partially cooked, add the onion and green onion and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried ingredients on top of the rice.
Add an egg yolk on top, if desired.