For the dressing, halve the lemongrass stalks widthways, then bash the thicker ends and finely chop the thinner ends. Place both in a small pan with the fish sauce, sugar, ginger, chilli and 5 tbsps water. Bring to the boil, simmer for 3-4 minutes then set aside to cool.
Heat a griddle or frying pan over a high heat. Put the chicken breasts between 2 sheets of cling film or baking parchment and flatten out with a rolling pin until about 1cm thick. Brush with ½ tbsp sesame oil and griddle for 4 minutes on each side until cooked through, the juices run clear and there is no pink meat. Slice into thin strips.
Meanwhile, put the noodles in a large bowl and cover with just-boiled water. Leave for 10 minutes, then drain and toss with the remaining ½ tbsp sesame oil. Divide between 4 plates with
the prepared vegetables and cooked chicken. Remove the thick lemongrass stalks from the dressing, stir in the lime zest and juice and spoon over the noodle salad. Pick the leaves off the Thai basil and, along with the crispy fried onions, scatter over to serve.