Wash the leeks and discard both ends, then thinly slice the white and light green parts.
Heat the butter in a saucepan. When it starts to foam, add the leek. Season with salt and simmer for 10 minutes under cover – stirring occasionally
Peel the potato and garlic, then thinly slice.
When the leek is soft, add the potatoes, garlic, rosemary – and three gratings of nutmeg.
Simmer for 15 minutes while stirring. When the potato is soft, mash gently with a masher then add the hot vegetable broth. Taste, and season with salt and pepper to your liking.
Bring to the boil, then add the chopped parsley.
Serve the soup with the cheese crumbled on top.