Put the ginger into a large, shallow pan with a litre of water and bring to the boil. Add the chicken breasts and leave them to simmer for 5–6 minutes. Turn off the heat and leave the chicken to cool in the liquid.
For the dressing, put the Thai fish sauce, vinegar, lime juice and sugar into a small pan and bring to the boil. Mix the cornflour with a teaspoon of water, stir this into the pan and simmer gently for 1 minute. Remove from the heat and leave to cool, then stir in the red chilli and garlic.
For the salad, peel the cucumber, cut it in half lengthways and scoop out the seeds with a teaspoon. Cut the flesh into 5cm-long matchsticks and add them to a large bowl with the spring onions, bean sprouts, mint and coriander, then toss together.
Lift the chicken breasts out of the poaching liquid and pull them into long chunky strips. Add these to the salad bowl and mix gently. Serve the salad with the dressing drizzled over the top and scattered with sesame seeds and chopped peanuts.
TIP: When peeling ginger, use the bowl end of a teaspoon to scrape the skin off. It’s much easier than using a peeler.