Seperate the light greeen tops from the whites of the leeks and whizz the green tops in a food processor.Thinly slice the white part of the leeks.
Heat the olive oil and butter over a medium high heat in a large saucepan. When the butter foams add the leeks and a good pinch of salt. Cover and cook for 10 minutes, stirring often, until softened.
Reduce the heat to medium and add the potatoes, garlic, nutmeg and rosemary. Cook uncovered for 20 minutes more, stirring often.
When the potatoes are soft and the leeks sweet mash well with a potato masher . Add the hot stock until the soup is as thick as you like it. Bring to the boil, season with salt and pepper and stir in the parsley.
Serve in bowls, crumbling the cheese over the top.