Slice the beef and onion thinly.
Heat cooking oil in a pan and stir-fry the beef.
When the beef is about halfway cooked, add the onion and stir-fry together.
Make the sauce by mixing soy sauce, sugar, mirin, and minced garlic.
Pour the sauce into the pan and simmer with the beef and onions.
Place the cooked beef and onions on top of the rice.
Garnish with green onions or sesame seeds to finish.