Heat the vegetable oil in a deep pot and sauté the red onion, tomatoes and green chili for two to three minutes.
Strain the tamarind pulp into the pot and add the rice wash. Bring this to a boil and then turn down to a simmer to cook the vegetables.
Add them to the pot according to how long they take to cook. Add the vegetables that take longer to cook first. As an estimate, the snake beans will take around 7 minutes, the daikon and eggplant around 5 minutes and the water spinach around 3 minutes.
Once the vegetables are done, add the prawns, which should take only around 3 minutes or so to cook.
Finally, add the fish sauce and sugar and adjust the seasoning to your liking.
