Heat 2 tbsp. oil and mustard seeds in a large nonstick frying pan over medium-high heat, covered, just until seeds start to pop. Stir in cumin seeds and turmeric; then stir in potatoes, 1 tsp. salt, and pepper.
Reduce heat to medium low and add 1 tbsp. water. Cook, covered, until potatoes are tender, 20 to 25 minutes.
Add remaining 2 tbsp. oil, coriander, cumin, cayenne, and salt to taste. Cook, uncovered, turning occasionally, until potatoes are golden, 8 to 10 minutes. Stir in cilantro.