Heat the oil in a wok or large saucepan over nearly high heat. Add the onions; cook and stir until browned. Mix in the curry leaves then season with the ginger garlic paste, coriander and salt. Cook and stir for 1 minute.
Season with salt and turmeric, then mix in the tomato, chilli powder, shrimp and water. Reduce the heat to medium-high and cook for 7 to 8 minutes until prawns are opaque. Taste and adjust salt and chilli powder if necessary. Season with garam masala, stir and remove from the heat. Garnish with fresh cilantro and serve with flat bread or rice.