Rinse the chicken and place in a large stockpot covered with cold water.
Bring to a boil while skimming scum, then add ginger and leeks.
Simmer for 1.5 to 3 hours, then strain and cool the broth.
Heat oil in a large saucepan and soften one chopped tomato for 3 minutes.
Stir in fish sauce, lemon juice, star fruit, and pineapple chunks.
Mix in pepper, tamarind, and sugar to taste.
Add 500ml chicken stock, spring onions, and the remaining tomato, then bring to a boil.
Boil for 2 minutes, add prawns, and cook for 1 minute until pink.
Remove from heat and stir in coriander and chilli before serving.