Wash the potatoes well and boil them with their skins on in plenty of salted water, with the lid on, until tender (about 20-25 minutes, depending on size).

Drain them, let them cool for a few minutes, and then peel them: this way, they will maintain a compact and well-bound consistency.

Meanwhile, cut the mozzarella into cubes, the cooked ham into strips, and the würstel into rounds.

Grease a not-too-deep baking dish (a glass dish is better, as it ensures even cooking and browning even on the bottom) and sprinkle it with breadcrumbs.

Form the first layer: mash 3 potatoes directly onto the bottom of the dish with a potato masher and level them with a fork, distributing them evenly over the entire surface.

Add the cooked ham, würstel, diced mozzarella, and some of the Grana Padano, distributing everything evenly. Finish with a few knobs of butter.

Now form the final layer of potatoes: mash the remaining potatoes over the filling, covering the entire surface well. Gently level with a fork, without pressing down too hard.

Finish with the remaining Grana Padano, the pieces of butter, and a light sprinkling of breadcrumbs. If you like, you can also add a drizzle of oil for a more intense golden colour.

Bake in a preheated static oven at 180°C, on the bottom rack, for about 30-35 minutes, until the surface is well-browned and crispy.

Turn off the oven and leave the gateau inside for another 10 minutes, then remove from the oven and let it rest for a few more minutes before serving.