First, make the paste. Dry-roast the coconut, cumin seeds and peppercorns until golden, then pound using a pestle and mortar or blend to a smooth paste.
Heat the oil in a saucepan, add the fenugreek and mustard seeds and cook until they start to crackle. Add the garlic, green chilli, ginger, curry leaves and onion, sauté until golden, then add the chilli powder, turmeric and coconut paste. Cook for 10-15 mins, making sure the mixture doesn’t stick – add a splash of water if you need to. Add the fish stock and reduce until the mixture thickens a little.
Add the coconut milk powder, tamarind pulp and tomatoes and stir in the drumstick leaves, if using. Season with salt and cook for 10 mins. Check the seasoning, then add the mixed crabmeat and heat through, stirring gently. Serve with rice.