Peel and cube yam, then light-fry in vegetable oil until golden crust forms; set aside.
Heat remaining oil, sauté chopped onions, ginger paste, and chopped green chili until soft.
Add chopped tomatoes, turmeric, red chili powder, coriander powder, and salt; cook until oil separates.
Stir in fried yam cubes, tamarind paste, and water; cover and simmer for 15 minutes until yam is tender.
Serve warm with rotis.