Heat a frying pan to medium then add the olive oil and onion. Cook, stirring occasionally, until softened (about 4 minutes).
Add the beef and brown, using a wooden spoon to break up any large clumps, until no pink remains (about 5 minutes).
Pour in the Bulgogi Sauce and simmer for 10 minutes, stirring occasionally and breaking up any small clumps of meat.
While the meat is simmering, halve and toast the buns, then butter them.
Pile just a little bit too much beef mixture on each of the buns, then top with a drizzle of sesame oil, handful of kimchi and sprinkle of spring onions.
Serve with chips, coleslaw or salad.