Bring water (1 or 2 cups) to a boil in a deep vessel. When it comes to a rolling boil, add enough salt and turmeric powder to it.
Add the crab to this and cook for 2-3 minutes. After that drain the water and keep it aside.
Heat the oil in a big kadai/pan and saute the onions till it turns brown in colour.
Add the ginger garlic paste and fry for a minute.
Next add the tomatoes, salt, chilly powder and coriander powder. (You can add few spoons of the water in which you cooked the crab to the masala to prevent the spices from getting burnt.)
Fry this mixture nicely till oil separates from the masala and most of the moisture has evaporated.
At this stage add the crab pieces, finely chopped curry leaves and coriander leaves.
Toss everything together for about 5 minutes till all the crab pieces are nicely coated with the masala.
Switch off. Tasty Nandu Varuval is ready to serve.