In a pan, add sliced potatoes, water until cover them, and let them cook for 15 minutes.
In a frying pan, add oil, chopped ham and let it fries. Next, add chopped onion and let it golden.
In another pan, add butter and let it melts. Add wheat flour and mix. Then, add milk, little by little, and mix it again. Add nutmeg, salt, grated cheese and keep stirring, until it gets the right point.
Transfer some of the cream to a refractory and spread well. Next, add the potatoes, chopped leek and ham. Finally, add the rest of the cream and spread well.
Finally, add sliced mozzarella, oregano and take to preheated oven at 356°F for 15 minutes.