If the pistachios are not pre-skinned, blanche them in a pan of boiling water for 2 minutes. Drain and then rub the nuts with your fingers, discarding the purple skins. Rinse in a bowl of hot water to remove any remaining bits of skin. Melt the margarine in a large saucepan and add the chopped onion, garlic, and leek. Fry for 10 minutes until soft. In a small bowl, mix the cornflour with 2 tablespoons of cold water. Toast the cumin seeds in a dry frying pan. Add to the fried onion mixture along with the cornflour paste, stock, salt, and pepper. Simmer for 15 minutes. Pour the soup and the pistachios into a food processor and blend for at least 3 minutes, until you have a smooth consistency. Return the soup to the pan and add the orange and lime juices. Heat through, then taste and adjust the seasoning to your preference. Serve with a swirl of cream, pomegranate seeds and some toasted pistachios.