1.Blend or process lemongrass, soy sauce, sugar and 1 tablespoon of lime juice until smooth.
2.Combine tuna and half the lemongrass mixture in a large bowl. Cook tuna on a heated oiled grill plate (or grill pan or barbecue), over high heat, 30 seconds each side or until cooked as desired. Remove from heat, cover; rest 5 minutes, then slice thinly.
3.Meanwhile, cook noodles in a large saucepan of boiling water until tender; drain.
4.Whisk remaining lemongrass mixture, oil and remaining juice in a large bowl, then add tuna, noodles and coriander; toss gently to combine. Season to taste.
You can use brown sugar instead of the palm sugar. Green tea soba noodles are flavoured with powdered green tea and are available from Asian food stores and in the Asian food section of most large supermarkets. Use plain dried soba noodles, if you like.