I once had chicory served in the following way but I bet it would be even better with leeks:
Poach some cleaned leeks till cooked. Poach whole if you think they'll be grit free, otherwise do as BreezyCooking suggests (cut in half, clean and then reconstruct). Butter a baking dish. Wrap each leek in a piece of ham. Lay side by side in the baking dish and cover with bechamel or cheese sauce. Sprinkle more cheese on top and then bake in the oven till browned.