Slice the monkfish into large chunks and pop in a non-reactive bowl, along with the turmeric, lime zest and juice and a large pinch of sea salt.
Mix together to coat the fish, then leave in the fridge for at least 1 hour.
Add the rice to a pan with 100ml of the coconut milk and 300ml of salted water, then cook according to the packet instructions.
Meanwhile, make your sauce. Peel and finely slice the onions and garlic, peel and finely chop the ginger, then slice the chillies. Roughly chop the tomatoes, keeping them separate.
Heat a large casserole pan over a medium–high heat and add a splash of oil, the onion, ginger, garlic, chillies and curry leaves. Cook for 5 to 10 minutes, or until the onion is softened and coloured.
Smash the cardamom pods in a pestle and mortar, then stir them into the pan along with the mustard seeds, cumin, fenugreek and turmeric. Fry for 1 minute.
Stir in the chopped tomatoes, tamarind paste or syrup, the remaining 300ml of coconut milk and 100ml of water, then simmer for 10 minutes, or until the tomatoes begin to break down and the sauce reduces.
Add the monkfish to the sauce and simmer until the fish is cooked through and opaque. Remove and discard the cardamom pods, then serve with the rice on the side.