Cut the dark green ends from the leeks, trim the bases and discard.

Slice the white part of the leek in half lengthways, then rinse well to dislodge any dirt or grit. Drain on a tea towel, then slice thinly.

Heat butter in a large heavy-based pot, add the leeks and cook over low heat for 15 minutes until tender.

Peel potatoes and cut into chunks. Add to pot and cook for a further five minutes, allowing them to soak up the flavours.

Add cheese rind and water and bring to the boil. Reduce heat and simmer for 40 minutes. Remove cheese rinds and discard.

Puree soup with a stick blender. Season well with salt and pepper to taste.

Place prosciutto under a grill or in a pan and cook until crispy. Crumble into rough shards, scatter over the soup with parsley and serve.