Heat oil on low heat and add cumin, fennel and fenugreek and saute for a minute. (Please note that if using powdered cumin and fennel, add them at the stage when adding the chilli and turmeric powder.)
Then add onion, ginger, garlic, curry leaves and stir till the onions are translucent.
Remove pan from fire and add the chilli powder, turmeric and coriander and return pan to fire, and keep the flame low.
Fry the masala and when it begins to dry, add the diced tomatoes.
When the tomatoes become pasty, add water to form a curry.
Add crabs and salt and coat the crabs with the masala mix, stirring occasionally till the crabs are cooked.