Place the tuna and lime juice in a shallow dish and leave to marinate for 10 minutes. Mix together the sesame seeds, chives, lemon thyme and chilli. Remove the tuna steaks from the lime juice and dip into the sesame seed mixture, making sure they are well coated. Place the soya sauce and water in a saucepan. Bring to the boil, then add the pak choi and cook for 4-5 minutes. Heat the vegetable oil in a frying pan, add the tuna and cook for 3-4 minutes each side or until cooked. (They should be firm but still moist.) Divide the pak choi between two warmed plates and spoon the remaining cooking juices over the top. Place the tuna on the pak choi and serve. NOTES : Fresh tuna steaks, cooked Oriental style, make a sophisticated meal for two. Quick to prepare and cook, the whole dish can be made in less than 30 minutes!