For the Dressing:
Carefully stir all the ingredients together in a bowl until the palm sugar has dissolved. Set aside and prepare the salad.
For the Salad:
Slice the green beans into 1cm lengths, then light blanch in boiling water, before immediately refreshing in a bowl of iced water. Remove and set aside. In a pestle and mortar, pound together the garlic and chilli to form a paste. To help the process, you can add a pinch of coarse salt.
Add the long green beans to the paste and continue to pound gently, followed by most of the crushed peanuts and dried shrimp (keep some aside to garnish). Add the green papaya juliennes and tomato wedges. You may need to scrape the contents of the bowl down then continue pounding.
Finally add in the dressing, mix together, taste and season with a pinch of salt if needed.
Serve by piling into a bowl and sprinkling on the reserved peanuts and shrimp. Add a sprig of coriander to garnish.
Notes: Green papaya salad is the most popular dish among women in Thailand. It is a Northeastern food that is eaten with sticky rice and other Northeastern dishes such as laarb, beef salad and bamboo shoot salad. For a vegetarian som tum, omit the dried shrimp and substitute soy sauce for fish sauce.
Recipe contributed by @Siam as part of the Senses of Thailand promotion at Selfridges.