In a skillet, add the unsalted butter and stir until fully melted.
Add the onion chopped into small cubes, stir-fry until slightly golden brown.
Add the wheat flour and stir until a mixture forms.
Gradually add the milk and keep stirring until combined.
Once you’ve done that, season with the salt, the nutmeg and the grated parmesan cheese, stir until incorporated.
Next, in a skillet, add 1 drizzle of olive oil and the onion slices, stir-fry until slightly golden brown. Set aside.
Now, in an ovenproof dish, add 1 drizzle of olive oil and spread it out evenly.
Add the potatoes chopped into round slices, covering the whole bottom of the dish.
Pour all the white sauce over the potatoes and spread it out again.
Add all the fried onion slices, the ham chopped into strips, and the grated mozzarella cheese.
Finally, top with the boiled eggs chopped into thin round slices, the grated parmesan cheese and the green olives.
Take to a preheated oven at 180°C (356°F) and bake for 35 minutes.
Serve.