Start by putting the potatoes into a saucepan and add enough water to cover them. Turn on the heat and let them boil for 20 minutes.
After this indicated time, remove them from the heat and chop them into smaller pieces.
Cut the ham slices into dices and set them aside.
Into a skillet, add a drizzle of oil and all the dices of ham, stir until slightly golden brown.
Add the onion chopped into rings and stir until golden brown. After that, set aside.
Now into another skillet, add the unsalted butter and stir until melted, add the wheat flour and stir for 2 minutes until cooked.
Gradually add the milk and keep stirring until a cream is formed.
Season with the nutmeg and the salt, stir well. Add the parmesan cheese and stir.
After that, into a baking dish, add 1 scoop of the sauce and spread it all over the bottom.
Add all the sliced potatoes, the slices of leek, the cooked ham and the rest of the sauce, spreading it well.
Top it all with the grated mozzarella cheese and bake in a preheated oven at 400 °F for 10 minutes.
Serve.