2 large Yukon Gold potatoes, peeled and cut into 1-inch cubes (about 4 cups)
2 tablespoons neutral oil, such as grapeseed or canola
1 tablespoon urad dal (split black gram)
½ teaspoon cumin seeds
¼ teaspoon black mustard seeds
2 tablespoons roughly chopped roasted cashews
1 green finger chile (or serrano chile), finely chopped
1 (1-inch) piece fresh ginger, peeled and finely chopped
1 medium yellow onion, chopped
½ teaspoon kosher salt, plus more as needed
¼ teaspoon ground turmeric
¼ cup chopped cilantro leaves and tender stems
½ lemon, for squeezing
Bring a pot of water to boil over high. Once the water boils, add the potatoes. Cook until tender, about 10 minutes, then drain in a colander.
In a heavy pot, heat the oil over medium. Add the urad dal, cumin and mustard seeds, and fry until cumin seeds are browned and dal is crisp, 1 to 2 minutes. Stir in the cashews, chile and ginger, and cook for another 2 minutes. Add the onion, salt and turmeric, and lower the heat. Cook, stirring occasionally, until the onion is translucent and tender but not browned, about 5 minutes.
Tip the cooked potatoes, half the cilantro and 1/4 cup water into the pot, and stir well to coat. As you stir, let some of the potato get mashed. If the mixture seems dry, add a splash more of water. Cook over low, stirring occasionally, until the potatoes are heated through, about 5 minutes, then season to taste with salt. Scrape into a serving dish; top with a generous squeeze of lemon and the remaining cilantro.