Slice the beef into bite-sized pieces.
Slice the onion thinly.
Heat oil in a pan and stir-fry the beef.
When the beef is half-cooked, add the onion and stir-fry together.
Add soy sauce, sugar, cooking wine, and pepper, then stir-fry until the flavors are absorbed.
Serve the stir-fried beef and onions over rice to finish.