Preheat pan to medium high heat with olive oil and add onions to cook until softened, about 5-7 minutes, stir often.
Remove onions from pan and leave to side. add more oil to pan and add in beef until beef is lightly browned. Add in sugar, soy sauce, mirin and stock. Bring to a simmer for 10 minutes until liquid becomes a thin sauce. Serve over rice with fried egg, scallions and sesame seeds!