Heat the oil in a large, nonstick wok and add the onion, garlic, ginger and chilli. Stir-fry for 1–2 minutes and add the curry leaves, curry powder, ground cinnamon and turmeric.
Stir-fry for 2–3 minutes and then add the coconut milk and stock. Bring to the boil, reduce the heat and simmer for 10–15 minutes, stirring occasionally.
Add the crab to the wok and simmer, covered, for 10–15 minutes. Remove from the heat and stir in the lemon juice. Season with salt and serve immediately with lemon segments, for squeezing.