• First toss the cod only with the turmeric, salt and lime juice and leave it for 10–15 minutes while you make the sauce. I wouldn’t marinate the prawns in lime because they’ll start changing texture.
• Heat the oil over medium heat and add the mustard seeds. Once they start popping, add the cumin and fenugreek. Give them about 20–30 seconds, then add the onion and a pinch of salt.
• Cook the onion for 6–8 minutes until soft and beginning to colour. Add the ginger, garlic and green chilli and cook another 1–2 minutes.
• Now add the coriander, turmeric and chilli powder. Stir for about 30 seconds, then add the curry leaves and your fresh lime leaves.
• Pour in the coconut milk, then add 1 tsp tamarind and ½ tsp sugar. Simmer gently, uncovered, for about 8–10 minutes. You’re looking for it to reduce slightly and lose that raw coconut-milk character.
• Taste it at this point. This is where I’d tune it. You want three things simultaneously: creamy coconut, a noticeable sour edge from the tamarind, and enough chilli that there’s a definite heat behind it. Add the second teaspoon of tamarind if it needs more bite.
• Then turn the heat down so the sauce is at the gentlest simmer.
• Nestle the cod pieces and the beans into the sauce and cook for about 4 minutes without stirring them around.
• Add the prawns, gently move the sauce around them, and cook another 2–3 minutes, until the prawns are just cooked and the cod flakes.
• Turn the heat off. Add the juice of the remaining half lime and most of the coriander. Taste for salt.
• Remove the lime leaves if they’re still in large pieces.

One little trick I’d definitely do

• Take one extra fresh lime leaf, remove the tough central rib and slice it absurdly finely — almost into green hairs. Sprinkle a small amount over each bowl with the coriander.
• That gives you two layers of lime: the deeper perfume from the leaves simmered in the coconut sauce and a really bright citrus hit from the fresh leaf at the end.