Slice leeks into 2 cm chunks and soak in cold water for 20 minutes, to remove any dirt from between the layers. Cut cooked ham into bite-sized pieces
and brown in a little hot oil. Remove and put into a deep oven-proof dish. Melt some butter in the pan, drain the leeks and sauté for 10 minutes,
covered, until softened. Add to the ham and mix gently. Make a thick cheese sauce by melting a Tbs butter with 2 Tbs plain flour and cooking the roux
for 2-3 minutes, then add about a cup of whole milk, stirring over a low heat until the mixture comes to a simmer and thickens. Add lots of grated
strong cheddar and stir until melted and the sauce is nice and smooth. You could make it extra tangy by adding a dollop of mustard, if you wish.
Pour this sauce over the ham and leeks. Now make some mashed potatoes (boil potatoes until soft, drain and mash with a good knob of butter),
and spoon this over the top, smoothing it out with a fork. Mix equal quantities of dry breadcrumbs and grated parmesan, and sprinkle this over
the mashed potatoes, then bake the whole dish in a 170°C oven for 30 minutes, until bubbling and the topping is browned and crispy.