Preheat the oven at 200°C. Wash the potatoes and make little holes on the skin. Spread olive oil all around and a bit of salt.
Bake for about 90 minutes. You don’t have to use a tray. The holes will avoid the potato to explode (I mean it!) and the even heat will bake it more quickly. The peel will become crunchy and the middle, quite soft.
While the potatoes bake, chop or grate the cheese and prepare the leek. Remove the tougher, darker leaves. Wash well, with the leaves down, letting lots of water all around, to remove all the dirt. Chop into thin slices and cook on a skillet with a bit of olive oil and salt. Let it cook for a while, stirring often. Remove from the fire when each slice is soft. Set aside.
Take the potatoes out of the oven carefully and put them on a tray. Cut each of them in half and remove part of the flesh from the middle. I used this to make another dish.
Add a little butter and a spoon of cream cheese to each half. Cover with the mozzarella and the leek. Bake for another 15 minutes, until the cheese has melted. Serve hot!