Tip the fish into a large bowl with add 1/2 tsp each of salt and plenty of freshly ground black pepper, half the lime juice and half the turmeric. Stir gently to combine. Set aside.

Heat the oil in a frying pan over a medium heat and cook the onion until softened, about 8-10 mins. Add the ginger, garlic, 1/2 tsp salt, the spices and remaining turmeric, and cook for 2 mins more until fragrant. Stir in the tomato purée and cook for a minute more, stirring. Tip the butternut squash and sweet potato mix into a heatproof bowl with a splash of water, cover and microwave on high for 3 mins until the veg is fork-tender. Tip the mixture into the pan with the onion and spices, and stir to combine.

Add the coconut milk, 180ml water, the tamarind paste, if using, and a pinch each of salt and black pepper, and simmer over a medium heat for 10-12 mins until thickened and fragrant.

Add the marinated fish to the hot coconut sauce in the pan and cook for 4-6 mins until the fish is opaque and cooked through. Break it into chunky flakes with a wooden spoon as it cooks. Remove the curry from the heat, stir in the remaining lime juice and scatter with the fresh coriander, then serve immediately with rice.