Simmer the chicken broth, tamarind paste, galangal and lemongrass for 15 minutes, strain the liquid into a new saucepan. Add pineapple, tomatoes, fish sauce and palm sugar and cook gently for 5 minutes. Then add the fish cubes and leave to stand for another 5-7 minutes. Mix in the garlic and sprouts, then slowly pour the egg into the boiling soup. Stir gently until the egg sets. (You may need to cook the egg in a little extra broth, this will keep the broth clearer.) Season to taste with pepper, mix in the Horapha and coriander.
Serve with the chopped chilli.
The soup should have a distinctly sweet and sour taste. Do not use canned pineapple, it will make the soup too sweet.