simple sweet potato curry
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1 large onion cut fine
3 small to medium Tomatoes cut fine
5 large Sweet Potatoes (Sakkaraivalikazhangu)  peeled and cut into 1/2 inch pieces (yielded approx 5 cups of cut potatoes)
1 tbsp oil
1 tsp cumin seeds
1/4 tsp turmeric powder (optional)
salt to taste
1/2 tsp red chili powder (or according to taste) 

Heat oil in a large heavy bottomed pan, add cumin seeds to it. When the seeds begin to sizzle add onions and saute until transparent. Add tomatoes and stir until the tomatoes are cooked and soft.

Add turmeric powder and stir a couple of times. Add sweet potatoes, salt to taste, red chili powder and mix well.
Cover with a lid and allow the sweet potatoes to cook on low to medium low heat, stir a few times in between to ensure even cooking. Make sure to cook the sweet potatoes until soft yet it retains its shape, over cooking can turn sweet potatoes into mush.  Keeping the pan covered usually provides sufficient moisture to cook the sweet potatoes, but keep an eye on it and if required, sprinkle a few drops of water for a little moisture to cook the potatoes.

Remove from heat and serve hot as a side dish to any Indian themed menu, this curry works very well with phulkas / rotis (Indian flat bread)  too. 

http://www.veginspirations.com/2010/10/simple-sweet-potato-curry.html