In a small bowl, mix together the BBQ sauce, chilli powder, sugar, soy sauce and cornflour to make the marinade. Add the steak, stir to coat and marinade for 20 minutes.
In a small bowl, mix together the BBQ sauce, oyster sauce and sesame oil to make the sauce. Set aside.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry the shallot and garlic until brown and aromatic, around 2-3 minutes. Add the steak and stir-fry until 70% cooked, about 2-3 minutes. Set aside.
Heat the same frying pan with 1 tablespoon oil over high heat. Add the onion and salt and fry until soft, 3-4 minutes. Stir in the kimchi and cook until the mixture gets dry, 2-3 minutes. Add the spring onions and steak and stir-fry for 1-2 minutes. Add the sauce, cover and allow to simmer for 2 minutes. Serve sprinkled with sesame seeds.
Kimchi is a traditional Korean side dish made from pickled vegetables.
You can buy kimchi and Korean BBQ sauce at Chinese/Oriental speciality stores or online.