First make the Bulgogi sauce: in a small saucepan bowl, combine the soy sauce, rice vinegar, brown sugar, gochujang paste, ginger and garlic. Bring to the boil and reduce it to a thick syrupy paste. This should only take 2-3 minutes. Add the sesame oil and let it cool. It keeps in the fridge for a few days.
Put the beef mince, coriander leaves and the spring onions into a bowl. Add the Bulgogi paste, salt and plenty of ground pepper. Using your hand, mix until well combined. Divide the mixture into 6 equal portions, shaping each into balls.
Flatten the balls into 2 cm thick patties.
Fry them in a griddle pan, on BBQ or grill them in the oven - on both sides. This should take about 5 minutes, depending on how well-done you like your beef.
Meanwhile, cut the buns in half and slightly toast them.
Make some pickled radish & cucumber, and you can mix some kimchi with mayo, or stir-fry some cabbage.
Assemble the burgers to you liking, then serve.