In a large soup pot, heat oil and sauté the onions, tomatoes, and ginger. Add the tamarind broth and bring to a boil. Reduce heat and allow to simmer for about 15 minutes. Add prawns, radish, and string beans. Simmer for 5 minutes then add the eggplant. When all the vegetables are tender, season with fish sauce and turn off the heat. Add kangkong leaves and stems. Serve hot.
Souring tip: If you like your sinigang very sour, use the same procedure but add 1/2 kilo more of tamarind fruit to make the broth. Once you have your broth after straining, reduce it on a low simmer. This will make a more concentrated broth that you can either store for future use, or use to make an extra tart sinigang.