Combine soy sauce, miso, ginger, sweet chilli sauce and chilli (if using) in a large plastic bag. Place steak in the bag and allow to marinate in the fridge for at least 2 hours or up to 24, turning the bag a few times, if possible, to redistribute the marinade. Sear the steak in a hot pan for approx. 4 minutes on each side, then rest for 5‒10 minutes before slicing and serving.