In a pan put the oil and warm it up (do not overheat it!) on a medium flame; once warm, reduce the flame to low and add the mustard seeds. After roughly 30 seconds to one minute, these will almost stop popping, and you will be able to add the cumin seeds.
After 30 seconds (or sooner if you see your seeds are browning very quickly), add the ginger and garlic; mix well and cook for about 30 seconds or until the smell of rawness of the garlic has gone. [Ensure these do not brown]
Bring the heat to medium and add the potatoes, onions, black pepper, green chilies, and salt; mix well and cook for 3-4 minutes, stirring occasionally.
At this point, when the onions are starting to soften on the edges, add the turmeric, coriander, salt, and chili powder; mix well and lower the flame. Keep on cooking this way until the spices are cooked, which is until when you can see the oil separating from them and the other ingredients, which should take about 2 minutes.
Mix the curry leaves, stir well for about 30 seconds, then add about ½ cup of water; bring to the boil and cover the pan with the lid.
After about 15 minutes, the potatoes will be cooked, the water will be evaporated and the curry will be ready.
Add the fresh cilantro/coriander, mix well, adjust the salt, and serve!