Finely chop the beef.
Finely dice the onion.
Heat some cooking oil in a pan and stir-fry the beef.
When the beef is cooked, add the onion and stir-fry together.
Season with soy sauce, sugar, and mirin.
Place the beef and onion mixture on top of rice.
Drizzle with mayonnaise.
Finish with chopped green onions and seaweed flakes.