Combine beef short rib, soy sauce, Japanese sake, and potato starch in a bowl. Mix well until the beef is evenly coated with starch. Let marinate for 10 minutes.
(Optional) If you want to top the raw egg yolks onto the rice, separate egg whites from yolks.
Heat 1 tablespoon oil in a large nonstick skillet over medium high heat until hot. Add beef short ribs. Let cook for 20 seconds without moving. Stir and cook until the beef is nicely browned on the surface. It’s OK if the beef is still a bit raw inside. Transfer to a plate.
Add 1 tablespoon oil into the same skillet and turn to medium heat. Add green onion. Stir a few times until fragrant.
Add gochujang. Stir a few times. Add rice. Stir and chop with spatula, until the rice is separated and coated with the sauce.
Move rice to one side of the skillet. Add the remaining 1 tablespoon oil to the empty side of the skillet and add the egg whites (if you don’t plan to top the rice with raw egg yolks, beat and add the entire eggs) into the oil. Let cook for a few seconds, until the bottom sets. Stir and cook to mix the rice with the egg.
Again move everything to one side of the skillet. Add bean sprouts and bok choy to the empty side. Stir and cook until the veggies turn soft.
Swirl in soy sauce and sugar. Return beef to the skillet. Stir and mix everything together.
Remove skillet from heat. Taste some rice and sprinkle a bit more salt, if necessary. Mix again. Transfer rice to serving bowls.
(Optional) Top egg yolks onto the rice. Garnish with green onion and sesame seeds.