Heat oil in a large skillet over medium heat.
Add mustard seeds and cumin seeds, and cook until they begin to pop.
Add the diced onion, ginger, garlic, and serrano chili. Sauté for 2-3 minutes until fragrant.
Stir in turmeric and salt, followed by the potatoes.
Pour in water, cover, and simmer on low for 15-20 minutes until the potatoes are tender.
Uncover and cook a few minutes more to allow excess moisture to evaporate.
Stir in chopped cilantro before serving.