Heat oil in a pan, add cumin, mustard, and allow them to splutter. Add hing, ginger and then the curry leaves. Fry together till you smell the ginger good. Do not burn.

Add onions, & green chilies. Fry till they turn golden.

Next add tomatoes and sprinkle salt, turmeric. Saute for 2 mins

On a low flame cover and cook till the tomatoes turn soft and mushy. The onion tomato mixture should blend well.

While the tomatoes cook, you can peel and cube the sweet potatoes, add them to salted water (2 cups water with ½ teaspoon salt) till we use.

Add chili powder, garam masala, coriander powder and red chili powder. Fry for about 1 to 2 minutes without burning.

Next add sweet potato cubes, pour the same water we used to keep them. Just use enough water to immerse the sweet potatoes. Bring it to a rolling boil, reduce the heat.

Cook on a low flame, till the sweet potatoes are soft cooked but not mushy. Taste test and adjust salt as required. If the curry looks runny, turn the flame to high and sook for a little longer so some of the water evaporates.

If you want you may add coconut milk at this stage (optional).

Keep covered for about 15 minutes for the flavors to absorb well by the sweet potato curry. Garnish with coriander leaves.