Marinade: In a bowl, combine all the ingredients.
Place the steak in a medium roasting dish. Pour over the marinade ingredients then use your hands to mix it all together. Cover and refrigerate for 3 hours.
Preheat the barbecue grill to medium. Grill the steak for 3–4 minutes on each side. Transfer to a plate then loosely cover with tinfoil. Set aside to rest for 10 minutes.
Pour the oil over the spring onion mixture and stir to combine. Set aside for a few minutes to cool down a bit then add the coriander, chilli flakes and a pinch of salt.