Heat the olive oil in a deep saucepan and saute the leek for 5 minutes.Add the parsnips and increase the heat slightly. Saute until they begin to soften then add the garlic and rosemary.
Increase heat to high and fry for 1 minute, stirring all the time,then add the stock. Bring to the boil then cover and cook over a medium flame until the parsnips are cooked.
Add the olives and cook for a minute more.
Serve in bowls, crumbling over the feta, and adding salt if needed.