Wash the sweet potatoes ver well, peel and dice in medium sized pieces.
Chop one onion into slightly thin and long slices.
Wash the mung beans and add it to a large pan.
Also add the diced sweet potatoes and chopped onion to the pan.
Add water and bring the water to a boil.
Once the water starts to boil, reduce the heat to low, cover the pan with lid and cook for about 25 minutes.
Meanwhile, chop the other two onions also in thin, long slices.
Heat the 2 tablespoons of oil in a different pan and add the mustards seeds.
Once the mustard seeds splutter (be careful), carefully add the chopped onions (if using cumin seeds instead of mustard seeds, they just need to lightly fry).
Saute the onions for about 2 minutes.
Add ginger & garlic, stir well and saute for another couple of minutes.
Add enough salt, stir & saute the onions until the onions are lightly browned, which will take about 8 to 10 minutes.
Now, dice the tomatoes in medium sized pieces and add this also to the pan containing onions.
Saute the mixture for about 5 to 6 minutes until the tomatoes begin to turn soft.
Add the tamarind paste and soy sauce, mix well and saute for another couple of minutes more.
Once the sweet potatoes & mung beans is cooked, add this onion tomato mixture also to the pan and mix well to combine with the sweet potatoes & mung beans.
Add all spice powders and also ghee or butter, mix well.
Simmer the contents for about a minute, then cover and cook the contents in low heat for another 4 to 5 minutes.
Serve hot with rice or Indian breads like chapathi, naan, poori etc or enjoy this warm as a delicious, rich and nutritious soup or stew!