Southern Indian Fish Curry

½ tbsp flavourless oil, e.g. groundnut, for frying
2 onions, peeled and finely sliced
2 tsp mustard seeds
1 tsp ground turmeric
2 tsp ground cumin
3cm piece of fresh root ginger, peeled and grated
1–2 long red chillies, deseeded and finely chopped, to taste
1 x 400ml tin reduced fat coconut milk
1–2 tbsp tamarind paste or watered-down tamarind block
1 small aubergine, cut into bite-sized pieces
2 carrots, chopped into bite-sized rounds
200g green beans, topped and tailed and cut in half
600g meaty white fish (e.g. cod, pollock, haddock or coley), cut into bite-sized pieces
Sea salt and freshly ground black pepper

TO SERVE

Place a large, shallow saucepan or a high-sided frying pan over a medium heat and add the oil. Once hot, add the sliced onions with a pinch of salt and sauté for 8–10 minutes, until completely soft.
Add the spices and continue to cook for a further minute or until you can really smell them, then add the ginger and chillies and stir over the heat for a further minute.
Pour in the coconut milk, tamarind paste and 400ml of water (use the empty coconut milk tin to measure the 400ml). Season with salt and pepper, stir well and bring to a simmer.
Once the sauce is simmering, add the aubergine and continue to cook for 5 minutes, then add the carrots and simmer for 10–15 minutes, until the carrots and aubergine are tender and the sauce has thickened a little.
Add the green beans and cook for a further 3 minutes, then add the fish. Stir well to coat, then cook for 3–4 minutes, until the fish is just cooked through. Taste and adjust the seasoning as necessary.
Serve the curry with rice in warmed serving bowls, sprinkled with toasted desiccated coconut, if using.