Peel and slice the potatoes into thin matchsticks. Soak in water for 10 minutes to remove excess starch, then drain well.
Heat oil in a pan on medium flame. Add cumin seeds (and mustard seeds if using) and let them crackle.
Add potatoes and mix to coat in oil. Cook uncovered for 2–3 minutes.
Add turmeric, red chili powder, crushed coriander, and salt. Stir gently.
Cover and cook on low heat for 10–12 minutes, stirring occasionally, until potatoes are tender.
Remove the lid, increase heat slightly, and stir-fry for another 2–3 minutes until lightly crisped.
Garnish with chopped coriander and green chili.
Serve hot with roti, paratha, or as a side with dal chawal.