Heat 4 tbsps oil in a large saucepan over a medium heat. Add the onion and cook for 15 minutes, taking care not to burn the onions, Spoon the caramelised onions out of the pan and drain on kitchen paper. Reserve half to garnish.
Pulse the other half of the onions to a thick paste in a blender or food processor.Add the stock and the spices and pulse to blend. Season to taste with salt and pepper. Set aside.
Heat the remaining 2 tbsps olive oil in a deep pan over a medium heat. Sear the fish on both sides. Add the rice to the pan and pour over the stock from the blender.
Bring to the boil, turn heat to low and cover and simmer for 12-15 minutes until the rice has cooked and absorbed all the liquid. Take the pan off the heat, cover and leave to rest for 10 minutes.
Spoon onto a serving dish , srprinkle with the toasted nuts and garnish with the reserved onions.