Thinly slice the beef.
Halve the onion, thinly slice 1/2 of the onion and chop the rest.
Chop the apple.
Put onion, apple, garlic, ginger in the food processor and blend until smooth. Put this in a large bowl.
Add the soy sauce, sake, sugar, sesame oil and gochujang in the same bowl.
Add beef and onion in the bowl.
Keep in the fridge overnight to 24 hours.
Heat the frying pan over medium-high heat.
Put sesame oil in the frying pan.
Place the beef one by one in the pan.
Cook until golden-brown for 1-2 minutes and then turn over.
Cook for another 1-2 minutes until both sides are golden-brown.