For the Curry Paste:
Deseed and soak the red chillies in salted water. Squeeze dry to extract the excess water. Chop and pound in a pestle and mortar until pureed. Add each ingredient in the order given, one at a time, and pound until a fine paste is achieved. Roast the spices individually, grind, sieve and stir into the wet paste.
For the Cucumber Relish:
Combine the vinegar, sugar, water, coriander root, pickled garlic and salt in a small saucepan and bring to the boil. Remove from the heat when the sugar dissolves. Cool, then strain. It should taste sour, sweet and a little salty, but it should not be too concentrated.
Mix remaining ingredients in the serving bowl and pour over the vinegar/sugar liquid. Serve with the curry.
For the Curry:
Bring the salted coconut milk to the boil. Add chicken, pandanus leaves and mandarin peel and poach until cooked - about 40 minutes. Remove and coarsely shred into quite large pieces. Strain and keep the stock.
Wash the potatoes then boil in salted coconut milk with the aromatics until cooked but still firm. Cool and peel. Cut in half before adding to the curry.
Meanwhile heat the separated and fresh coconut cream, add the curry paste along with the additional spices for at least 15 minutes until fragrant and quite oily. Add some of the oily part of the chicken poaching liquid. Season with sugar and fish sauce and continue to simmer for a few more minutes.
Moisten with the chicken poaching liquid, ladle by ladle, until a medium-thick curry is achieved. Add the chicken and potatoes (reheat the spuds in the chaffing pot and so reduce any starch from thickening the curry).
Check the seasoning: it should taste rich, salty and just a little spicy. Finish with the chilli, galangal and pepper powder and a good squeeze of mandarin juice. Serve napped with the thickened coconut cream and sprinkle with deep-fried shallots and a cucumber relish.
Recipe contributed by Nahm as part of the Senses of Thailand promotion at Selfridges.