Heat the oil in a large saucepan, add the onion and cook for 5 minutes or until soft. Add the parsnips, potatoes, apples and rosemary and stir to heat through. Make up 1 litre of stock and pour into the saucepan. Bring to a simmer. Cover and cook for 25 minutes or until the vegetables are tender.
Use a hand blender to purée the soup until velvety smooth. Stir in the milk and heat through. Set aside half of the soup for freezing.