7 large boiling potatoes, 3 1/2 to 4 lbs. total
3 tablespoons extra virgin olive oil
4 tablespoons unsalted butter (plus butter to line the baking dish)
2 medium yellow onions, peeled and thinly sliced
Salt to taste
1/4 pound sliced prosciutto or cooked ham, cut into short strips
1/4 pound smoked mozzarella, grated or shredded
2-3 tablespoons freshly grated parmigiano cheese
2-3 tablespoons unsalted butter, cut into small pieces
Put the potatoes in a large pot and cover with cold water. Bring the water to a boil and cook, uncovered, over medium heat until potatoes are tender but still a bit firm. Drain the potatoes and set aside to cool. Peel potatoes and cut them into 1/4-inch-thick rounds.
Meanwhile, heat the oil and two tablespoons of butter in a skillet over medium heat. When the butter begins to foam, add the onion and cook for five to six minutes, stirring continually, until it is lightly golden and soft. Scoop up the onion and set aside.
Preheat the oven to 400 degrees. Generously butter a medium-sized baking pan.
Line the bottom of the pan with potato slices, slightly overlapping each other, and season lightly with salt. (Don't overdo it, as the smoked mozzarella is already salty.)
Spread about half of the onion and prosciutto over the potatoes, then sprinkle half of the mozzarella. Repeat with one more layer of potatoes, onions, prosciutto and smoked mozzarella. Top with a light sprinkling of parmigiano and dot with remaining butter.
Put the pan on the middle rack of the oven and bake until the top has a nice golden color, 15 to 20 minutes. Remove from the oven, cool for about 10 minutes, and serve.