Peel and roughly chop the onions, crush them in a blender or in mortar and pestle.
Finely chop the tomatoes and coriander leaves. Keep it aside.
Remove the drumstick from stalk, clean in running water, drain the water and keep it aside.
Grind the grated coconut along with 1 ½ tsp of cumin seeds to a fine paste by adding water.
Heat oil in a heavy bottom and wide mouth kadai, add in mustard seeds, leftover cumin seeds, fenugreek/methi seeds and wait till they crackle.
Add in the drumstick/moringa leaves and cook until the leaves turn dark green in color.
Add in the onions and ginger-garlic paste and cook until the raw smell goes off or the onions turn to golden brown in color.
Add in the tomatoes, cook until they turn mushy and the oil separates along the sides.
Add in the spice powders (turmeric, chili and coriander powder) and cook until the raw smell goes off.
Pour in a cup of water, bring it to boil and now pour in the grounded coconut paste, bring it to boil for few mins or until the raw smell goes off.
Add in two pieces of crabs, cover and cook for few mins or until the oil floats on the top.
Add in all the other crab pieces, cover and cook for few mins or until the crab is cooked (the crab changes to blue color).
Remove the lid and keep in medium flame, until the curry thickens.
Serve hot. Goes out well with steamed rice for lunch.