Cool arbi slightly, peel and cut into half-inch thick slices. Take yogurt in a bowl. Add water chestnut flour and whisk well till properly blended. Add rock salt, red chilli powder and turmeric powder and mix again. Add two cups of water and whisk well.
Heat sufficient oil in a kadai and deep fry arbi till golden brown.
Drain and place on an absorbent paper. Set aside. Heat two tablespoons oil in a deep pan. Add cumin seeds and dry red chillies.
When cumin seeds begin to change colour, add the yogurt mixture. Cook for five minutes, stirring continuously.
Add the fried arbi pieces and mix. Continue cooking for five more minutes or till you get the desired consistency.
Garnish with coriander leaves and serve hot.