Using ground beef instead of thinly sliced beef in bulgogi is an easier version, with all the authentic flavors intact. Though Gochujang is not part of bulgogi, I would recommend adding it.
Thaw the ground beef if its frozen. Drain any liquid/water well.
To ground beef, add grated apple, gochujang, soy sauce, dark soy, mirin, ginger, sugar, sesame oil and sesame seeds. Mix well and rest for 15-20 minutes for the beef to tenderise.
Stir Fry
Heat oil in a wok. Add sliced onion, part of green onion, and minced garlic. Saute for a minute.
Keep the heat medium-high. Add marinated ground beef to the wok and spread it in a single layer. ( spreading it in a single thin layer, helps browning the meat quicker than steaming it, hence using a wok or a large skillet works best).
After cooking it undisturbed for 3-4 minutes, flip, and start breaking big lumps of meat. Keep cooking on medium-high heat for another 6-7 minutes until most of the liquid has evaporated and beef is cooked through.
Finish it with more toasted sesame seeds and green onion.
Notes
I have used sliced onion instead of grated onion in the marinade because grated onion will introduce lots of liquid to the ground beef and cooking will take longer. Also, sliced onion gives a bit of texture.
Gochujang is not traditionally used in bulgogi, but I love the extra depth of flavor. You can skip it.
If you can get hold of Korean Pear, use it instead of apple.