Preheat the oven to 375° and line two sheet pans with parchment paper. Place the potato in a microwave safe bowl and pierce with a knife. Drizzle with the oil and cook in the microwave until soft, flipping halfway through, 7 to 9 minutes. Let the potato cool, then peel and mash. You should have 1½ cups of mashed potato.

To the mashed potato, add the quark (or yogurt), feta, half (1½ tablespoons) of the whisked egg, salt and pepper. Mix until evenly combined; you should have about 2½ cups of filling.

On a cutting board, spread 1 of the 4 sheets of puff pastry (each should be a 9-inch square) and cut into 6 even rectangles.

Take each piece and stretch it carefully in your hands into a 3-inch square. Place 1 tablespoon of potato-cheese filling in the middle of the square, and close the dough from one diagonal corner to the opposite, creating a triangle. Close the open ends lightly to allow proper rising.

Place the triangle on the prepared sheet pan and continue with the rest of the dough and filling until you have 24 bourekas. The bourekas can be placed right next to each other, as they won’t spread out to the sides. Each pan should hold 12 bourekas.

Using a pastry brush or your fingers, brush the remaining beaten egg over the bourekas then sprinkle the tops evenly with sesame seeds.

Place the pans in the oven and bake, rotating halfway through, until golden brown, 30 minutes. Serve immediately or at room temperature.