Wash the potatoes well. Do not peel them. Cut them in half if they're small, into quarters if large.
Heat the oil in a skillet over medium heat. Add the cashews and cook until browned, 2 to 3 minutes. Remove the nuts from the skillet and set them aside.
In the remaining oil, cook the onion, ginger and garlic over medium-low heat until very tender, about 5 minutes. Stir in the curry powder, turmeric and salt. Add the potatoes and toss with the seasonings.
Add the water and yogurt, cover and cook over medium heat until the potatoes are tender, 20 to 25 minutes. If the heat is too high and the potatoes dry out, add water, a tablespoon at a time. There should be no excess liquid remaining in the pan.
Stir in the cashews, then the cilantro. Turn the salad out onto a platter or into a bowl. Serve at room temperature.