Step 1

Make the crust: Pulse the flour, sugar and salt in a food processor.

Step 2

Add 1 stick butter and pulse until combined.

Step 3

Add the remaining 2 sticks butter and pulse three times, or until the mixture resembles coarse meal.

Step 5

Heat 2 tablespoons olive oil in a skillet over medium heat.

Step 6

Add the onion and cook until slightly brown, 5 to 7 minutes; season with salt and let cool. Slice the potatoes and apples very thinly; toss in a bowl with the thyme, the remaining 2 tablespoons olive oil, and salt and pepper to taste.Line an 8-inch square baking dish with foil, leaving a 1-inch overhang on all sides. On a floured surface, roll out one piece of dough into a 12-inch square, about 1/8-inch thick.

Step 7

Transfer to the prepared dish, tucking the dough into the edges and corners; do not trim the overhang.

Step 8

Sprinkle the breadcrumbs over the dough and add the onion.

Step 9

Lay the mortadella on top, then sprinkle with about one-third of the cheese.

Step 10

Layer half of the potatoes and apples on top in a single layer, then sprinkle with another one-third of the cheese. Top with the remaining potatoes, apples and cheese.

Step 11

Roll the remaining dough into a 12-inch square and place on top of the filling. Press the overhang of the two crusts together, then roll the dough under itself and pinch to make a 3/4-inch-high edge. Make slits in the top crust with a knife to allow steam to escape. Refrigerate 30 minutes. Meanwhile, preheat the oven to 425 degrees F.

Step 12

Bake the pie until golden brown, about 30 minutes. Reduce the oven temperature to 400 degrees F, brush the crust with cream and continue baking until crisp, about 35 more minutes. Cool 25 minutes, then lift out of the pan and slice.