Preheat the oven to 200ºC / 400ºF / Gas Mark 6. Rub the oil all over the potatoes, place on a
baking sheet and bake in the preheated oven for 1 hour, or until the flesh is soft.
Remove the potatoes from the oven, cut in half lengthways and carefully scoop out the flesh into
a bowl, keeping the skins intact. Set the skins aside.
Add the milk, butter, Cheddar cheese and egg yolk to the potato and mash well. Season to taste
with salt and pepper. Mix in the ham.
In a separate, grease-free bowl, whisk the egg white until stiff, then fold into the potato
Pile the potato mixture back into the skins and sprinkle over the Parmesan cheese. Return to the
oven and bake for 20 minutes. Serve with salad.