Fry some onions in oil. Add ginger garlic paste. Chopped tomatoes and saute till oil separates. Add cleaned crabs and mix it well. Cover and cook on medium heat for 10 mins.
Add red chilly powder, turmeric powder, roasted coriander+cumin powder, fennel powder. Add a little water and cook till its almost done. Make a powder of black pepper and fresh coriander. Add it to the dish. A teaspoon of vinegar just before you switch off the stove.
Enjoy at room temperature so its easier to grip and break the shells.