Pour the tamarind paste and water mixture through a sieve into the stockpot. With a spoon, press down on the tamarind flesh to extract as much pulp as possible. Discard the seeds and residual pulp.
Stir to combine and bring to a boil.
Add the fish fillets to the broth and cook for 5 minutes. Remove the fish and reserve.
Slice the pineapple lengthwise, then cut each piece into 1/2-inch (1.2 cm) thick slices.
Add the pineapple and tomatoes to the broth and bring back to a boil.
Add the okra and taro, lightly season with salt and pepper, and keep cooking for 2 minutes.
Add the fish back to the pot, then stir in the bean sprouts, rice paddy herbs and cilantro. Bring back to a simmer, then remove from heat.
Serve piping hot, sprinkled with fried onions or garlic, with bowls of bird’s eye chili and fish sauce dressing to dip the fish in.
Notes
The longer you let the flavours infuse, the better the soup is. You can prepare it hours in advance and reheat it when you're ready to serve.
Garnish with reserved cilantro and green onions just before serving.