Preparation

Make the filling

• Heat the oil in a large skillet over medium heat. Add the onion, and cook until deeply browned, about 10 minutes. Add the rosemary and garlic, and cook until fragrant, 1 minute. Season to taste with salt and pepper.

Make the pie

• In a large saucepan, cover the potatoes by 1 inch with well-salted water. Bring to a boil over high heat, then lower to a simmer. Cook the potatoes until tender, about 15 minutes. Meanwhile, coat the bottom and sides of a 9-inch springform pan with 1 Tbs. of the butter, then cover evenly with 1/4 cup of the panko.
• Position a rack in the center of the oven, and heat the oven to 400°F.
• Drain the potatoes well, and return them to the pot to steam, about 2 minutes. Add 2 Tbs. of the butter, the cream, eggs, Parmigiano, 1-1/2 tsp. salt, and 1/4 tsp. pepper, and mash thoroughly.
• Spread half of the mashed potatoes evenly in the prepared pan. Cover with 1 cup of the scamorza, followed by the onion mixture and the remaining scamorza. Top with the remaining potatoes, and smooth the top with a silicone spatula.
• Sprinkle the surface with the remaining 1/4 cup panko, and dot with the remaining 1 Tbs. butter. Bake until the top is golden-brown and the center is very hot, about 45 minutes. Let cool for 5 minutes before running a paring knife around the edge to release the pan. Cut into wedges and serve with a green salad, if you like.