Heat the oil in a wok or pan and add the mustard seeds. When they splutter, add the minced ginger, curry leaves and chopped green chillies, and sauté for a minute. Turn the heat to medium and add the asafoetida and the turmeric powder and stir a couple of times. Then add the mashed potato. Add the required salt and keep stirring everything till well mixed. Take it off the heat and let it cool to room temperature.