Clean the potatoes and peel them if necessary. New potatoes only need a light scrub. Wash them, cut into small cubes, and set aside.
Trim the green top of the leek (the dry part), as it won’t be needed. Cut off the root end as well. Remove any damaged or very dirty outer leaves. The inner layers of leek often contain sand, so wash thoroughly, then slice into half rings. Set aside.
Melt the butter in a pot, add the potatoes, and sauté them. Once they start to get a bit of color, add the leek. Season with salt and pepper, and sauté together.
Pour in the water. Add a bit more salt and bring to a boil.
Once boiling, reduce the heat, pour in the cream, and cook until the potatoes are tender.
Add the grated Parmesan and a pinch of freshly grated nutmeg. Cover and let it simmer for another 5 minutes, then serve hot.
For the topping, bake the diced bread and ham on a baking tray lined with parchment paper until crispy. Serve on top of the soup.