To start, separate the leaves and stems of the coriander. Place the leaves aside for later.
Add the coriander stems, onion, garlic gloves and ginger to a food processor or blender. Blitz until a chunky paste has formed.
On medium heat, warm the olive oil in a large, deep-sided frying pan. Once hot, add the mustard seeds and cumin seeds. Cook for a couple of minutes, until fragrant.
Add the paste to the pan and cook for a minute, mixing occasionally.
Incorporate the dried spices and lime zest, stirring constantly while cooking for a further 2 minutes.
Pour in the tomatoes, tamarind paste, brown sugar and 250ml water. Cook for 5-10 minutes until the sauce has reduced slightly.
Meanwhile, chop your cod loins into bitesize chunks.
Add the cod pieces into the sauce and season to taste. Partially cover the pan with a lid and cook for 5 minutes, until the fish is just cooked.
Remove from the heat and stir through the coriander leaves. Serve immediately with steamed Basmati rice and tuck in!