Put the rice on to cook as per the packet instructions. Heat the oil in a large frying pan and gently fry the onion for around 8 minutes until soft. Add the garlic and fry for a further 2 minutes.
Add the curry paste to the onions and stir for 1 - 2 minutes.
Add the tinned tomatoes and vegetable stock.
Bring to a simmer, season with salt and freshly ground black pepper and add the fish. Continue to simmer for 4 - 5 minutes until the fish is cooked through and flakes easily. Don't over stir it or the fish will break into tiny pieces.
Stir through the baby spinach one handful at a time.
Stir in the natural yoghurt if you like and serve with the rice or naan bread.