Preheat the oven to 400°F (200°C), Gas mark 6. Blanch the leeks in a large pan of boiling salted water for 3-4 minutes until just tender. Drain and quickly refresh under cold running water to stop them from cooking any further, then drain well and pat dry on kitchen paper.
Generously butter a baking dish, just large enough to hold the leeks. Wrap each leek in two slices of the Parma ham and arrange them in the buttered baking dish. Place the cheese in a bowl and stir in the Dijon mustard and crème fraîche until well combined. Season to taste. Spread over the leeks and bake for 15-20 minutes until bubbling and golden brown.
Serve the dish of leeks wrapped in Parma ham with cheese straight to the table with plenty of crusty bread to mop up all of those delicious juices and a separate plate of tomato salad.