Put the oil in a large, high-sided pan over a medium heat. Add the onion and fry for 5 minutes. Reduce the heat and add the curry paste and turmeric; cook gently for 5 minutes more until fragrant. Add the garlic, sugar, fish sauce, tamarind, and the lime zest and juice. Cook for 2-3 minutes, until the sugar dissolves and everything is sizzling.
Add the coconut milk and stock; boil for 3 minutes then simmer briskly for 15 minutes. Stir through the spinach and prawns; cook for 2-3 minutes, until the prawns are pink throughout. Season as needed.
Meanwhile, cook the rice noodles according to pack instructions. Divide the curry into bowls, then top with the noodles, onion and chilli. Serve immediately, with the lime wedges for squeezing over.
