Place all the ingredients for the curry paste in a mini blender and blend until smooth, adding a little water if necessary. Transfer the paste to a wide saucepan and add the fish sauce, sugar, lemon grass stalks and two-thirds of the measured water. Bring to the boil, reduce the heat and simmer for 8–10 minutes.
Mix the lemon juice with the tamarind paste and remaining water. Add to the saucepan with the pineapple and stir to mix well. Add the salmon fillets and simmer gently for 8–10 minutes until cooked through. Remove from the heat and serve immediately with steamed jasmine rice.