Preheat the oven to 190º C, gas mark 5. Have ready a large shallow oven-proof dish.

Cut the leeks in half across their width. Par-boil for 10 minutes.

Drain the leeks well. Split them half open along their length. Reserve 50g of Cheddar Cheese, cut the remainder into 8 long batons. Use to fill each opened leek.

Roll the stuffed leeks in a slice of ham and arrange in the dish.

Melt the butter in a saucepan and beat in the flour and mustard, cook for one minute. Add the milk whisking continuously until the sauce thickens. Remove from the heat and season with salt and pepper.

Pour the cheese sauce over the leeks. Grate the reserved cheese and sprinkle over the sauce.

7. Bake for 20-30 minutes until the leeks are tender and a golden brown crust has formed. Serve immediately.