In a frying pan with the heat turn off, combine beef stock, sugar, and soy sauce. Add slices of onion and spread them throughout the pan, then add the beef slice on top until the beef has covered the onion. Cover the pan with a lid and cook over medium heat.
Let it simmer until the beef is fully cooked and the sauce is absorbed.
Scoop rice into a bowl, put the beef on top, drizzle it with the sauce, then arrange the egg on top, and garnish with scallion. Enjoy!