Place the vegetables, stock, garlic and coriander sprig in a large pan. Bring to the boil, cover and simmer for 45 minutes until the vegetables are tender. Liquidise and re-heat gently. Meanwhile, heat the ghee or vegetable oil and fry the onion for about 10 minutes, stirring constantly until golden. Stir in the curry powder and flour and stir fry for a minute. Add this mixture to the soup. Simmer the soup until it is thickened, then add the cream and chopped coriander leaves.