Cut potatoes into 1/2-1 inch cubes, and chop onions finely.
Meanwhile, in a small frying pan, heat oil on medium. When it is fully heated, add mustard seeds and let them pop. When they finish popping, add hing (or garlic), cumin seeds, green chilies, and let it cook in oil for 1 minute or until slightly browned.
Add in onions and sautee in spices. Once translucent, add in potatoes, turmeric, coriander powder and mix well.
Cover the pan and add some water. Close the lid and let potatoes cook. Stir potatoes intermittently so they don't stick to the pan.
Squeeze lemon juice at the end and garnish with chopped cilantro. Enjoy!