Heat the oil in a large frying pan and fry the onions for 5 minutes or until they are soft. Add the curry leaves, green chillies, garlic and ginger and cook for 5 minutes over a medium heat.
Add the chilli and turmeric powders to the pan then pour in 225 ml water, stirring slowly. Bring to the boil then reduce the heat and simmer for 10 minutes, stirring occasionally.
Add the crab pieces and continue cooking the curry over a medium heat for 10 minutes.
Stir in the coconut milk and allow the curry to heat through gently for 2-3 minutes. Add salt to taste. Remove the pan from the heat and set aside for a few minutes. Pour in the fresh lemon juice and serve straight away.