Heat oven to 200C/fan 180C/gas 6 and boil the kettle. Place the leeks in a single layer in a large saucepan and add enough hot water from the kettle to cover. Bring to the boil, then cover and simmer for 5 mins until the leeks are just tender.
Lay 2 slices of ham, overlapping, on a board, then sit 2 leek halves at one end. Wrap up, repeat with the other leeks, then place the parcels in a small ovenproof dish.
Mix three-quarters of the cheese with the creme fraiche, then season and spoon over the parcels. Mix the remaining parmesan with the breadcrumbs and scatter over the top. Bake for 15 mins until the cheese is golden and bubbling. Serve with a green salad and crusty bread to mop up the sauce.