Slice the onion and green onion thinly.
Season the beef brisket with salt and pepper.
Add cooking oil to a pan and stir-fry the brisket.
When the brisket is half cooked, add onion and green onion and stir-fry together.
Add soy sauce, sugar, and mirin to make the sauce.
Place the stir-fried brisket on top of rice.
Add seaweed flakes and egg yolk on top to finish, as desired.