Step 1In a large saucepan, heat the oil over medium heat. Add the onion and cook gently, stirring occasionally, for about 4 minutes, or until fragrant and soft. Add the salt, sugar, fish sauce, tamarind liquid and 5 1/2 cups water.
Step 2Bring to a boil, lower the heat to a simmer, add the catfish and the pineapple, and simmer for 5 minutes, or until the catfish and pineapple are tender but still firm. If you are not serving the soup right away, turn off the heat and cover.
Step 3Just before serving, return the soup to a simmer. Drop in the okra and cook for 2 minutes, or until tender but still firm. Add the tomatoes, bean sprouts and cumin. When the bean sprouts have just wilted, after about 30 seconds, turn off the heat.
Step 4Taste and add extra salt or fish sauce, if necessary. Ladle into serving bowls and garnish with the cilantro.