Melt the butter in a pan and sweat the onion for 10 minutes. Add the pumpkin, cumin, stock, and salt and pepper to taste. Bring to the boil, cover and simmer gently for 30 minutes. Process in a processor or blender and adjust seasonings. Return the soup to the pan, reheat to serve, and float a dessertspoonful of soured cream on the surface of each bowl.