Cut the beef into bite-sized pieces.
Slice the onion thinly.
Add oil to a pan and stir-fry the beef over high heat.
When the beef is about halfway cooked, add the onions and stir-fry together.
Make the sauce by adding soy sauce, sugar, and cooking wine.
Pour the sauce over the beef and onions and simmer.
Serve the beef, onions, and sauce over the rice.
Top with an egg yolk and chopped green onions to finish.