Thinly slice the beef and onion.
Add oil to a pan and stir-fry the beef.
When the beef is about halfway cooked, add the onion and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the cooked beef and onions on top of rice.
Drizzle with mayonnaise and top with chopped green onion and seaweed flakes as desired.