2 spring onions, thinly sliced diagonally (green end only)
1 tbsp sesame seed, toasted
Put the soy sauce and brown sugar in a small saucepan over a medium heat and stir until the sugar dissolves. Add the garlic oil and sesame oil, remove from the heat and cool. Cover the tuna steaks with the marinade and leave for 1 hour.
Barbecue the tuna steaks for 1 minute each side for rare, 2 for medium, basting constantly with the marinade, until charred and sticky on the outside.
Barbeque the bok choy halves cut-side down for 2 minutes. Put in a bowl and squeeze over lime juice and a little oil and scatter with spring onions and sesame seeds. Serve with the tuna.