A restaurant dish and probably a hybrid of some of India's more popular homegrown potato recipes. I have to say this dish beats my own Punjabi cumin potatoes, and it goes with almost everything.
Bring a large pot of salted water to a boil. Add the potatoes and boil until just tender, up to 30 minutes. Drain. When cool enough to handle, peel and chop into 1-inch cubes.
Quarter 1 of the tomatoes. Blend together the ginger, garlic, and quartered tomato until smooth.
Heat the oil in a large nonstick skillet. Add the cumin and mustard seeds, and once the cumin starts to darken, add the onion. Cook for 1 minute before adding the ginger and garlic mixture, the ground spices, and salt, to taste. Sauté gently until the garlic smells cooked, for 1-2 minutes.
Cut the other tomato into slim wedges. Add the tomato wedges, stir well, and cook for 3-4 minutes. Tip in the potatoes and cook for 3-5 minutes to absorb the flavors. Taste and adjust the seasoning, stir in the chopped cilantro, and serve.