Cut each stick of lemon grass 10cm from the root end; discard tops. Peel away tough outer layers. Split lemon grass in half, remove V-shaped core and discard. Chop remaining lemon grass finely.

Place noodles in a medium heatproof bowl, cover with boiling water. Stand until tender; drain. Rinse under cold water; drain.

Meanwhile, to make dipping sauce: combine sugar and water in a small saucepan. Stir over low heat, until sugar dissolves. Bring to the boil; reduce heat. Simmer, uncovered, 2 minutes or until the sauce thickens slightly. Remove from heat; add vinegar and chilli.

Heat a little of the oil in a wok or large frying pan; stir-fry chicken, in batches with oil, until browned and cooked through. Return chicken to the wok; make a well in the centre. Add the remaining oil, lemon grass and garlic. Cook 1 minute, or until fragrant and softened. Add sauce; toss to coat chicken in sauce.

Divide noodles among serving bowls. Top with lettuce, carrot, cucumber and chicken. Sprinkle with peanuts and radish. Serve with sauce.