Peel the onion and slice in medium sized slightly thick pieces, keep this aside.
Peel the potatoes and cut in small to medium sized cubes, keep aside with the chopped onions.
Heat oil in a medium sized pan, add mustard seeds.
When the mustard seeds splutter, add dried broken red chilies, curry leaves and fry for a few seconds.
Add the potatoes and onions together to the pan, stir well and lightly fry for about 2 minutes.
Add enough salt, ginger and garlic and also green chilies to the pan, stir well and lightly fry for another 1 more minute.
Add the water, turmeric powder and red chili powder also to the pan, stir everything well and bring the mixture to a boil.
Once the mixture starts boiling, cover the pan with lid and cook the contents in low heat for about 8 to 10 minutes or until the potatoes cooked and softened and the gravy is thickened. You may add a little more water if needed, depending on how much gravy you need. The thickness of this gravy comes from the potatoes and as you cook the potatoes more, the gravy thickens.
Serve the hot potato curry with poori or chapathi or even serve this as a side dish with dosa or idli as you wish.