Instructions

• If using a raw russet potato: preheat oven to 400°F. Scrub and dry a 9-ounce russet potato, prick it a few times with a knife, place it on a baking sheet, and bake 60–70 minutes until very soft when pressed. Remove from oven, let cool until you can handle it, peel, and mash. You should have about 1 cup of mashed potato. If you already have 1 cup cold mashed potatoes, skip this step.
• Preheat the oven to 350°F. Position two oven racks so you can switch baking sheets between an upper and lower rack during baking.
• In a mixing bowl combine 1 cup cold or room-temperature mashed potato, 1/4 cup crumbled feta cheese, 1/4 cup shredded kashkaval cheese (or substitute an additional 1/4 cup feta), 1 large egg, and a pinch of salt and a pinch of black pepper. If your mashed potatoes are already seasoned or you used only feta, you may omit the added salt. Use a fork to mash and mix until well blended and any large feta crumbles are broken up. Set the filling aside.
• Thaw puff pastry sheets according to package directions until they are pliable but still cold. On a lightly floured, smooth work surface, unfold one sheet and roll it to a 12" x 12" square.
• Cut the 12" x 12" sheet into 9 equal squares (about 4" x 4" each). Repeat with the second sheet so you have 18 squares total.
• Place about 1 heaping tablespoon of the potato-cheese filling on one corner of a pastry square. Spread the filling into a triangular shape, keeping the filling about 1/4" from the pastry edges and filling roughly halfway across the square.
• Fold the opposite corner of the square up and over the filling to form a triangle. Press firmly along the open edges to seal. Optionally crimp the sealed edge with the tines of a fork. Repeat for all squares.
• Prepare one or two baking sheets by spraying with nonstick cooking oil spray or lining with parchment paper. Arrange up to 9 bourekas per sheet, leaving space between them for expansion.
• In a small bowl, whisk the 1 large egg yolk until smooth. Lightly brush the tops of each boureka with the egg yolk. If using topping, sprinkle up to 1 tablespoon sesame seeds or poppy seeds evenly over the bourekas.
• Bake the bourekas at 350°F for about 35 minutes, switching the baking sheets between the upper and lower racks halfway through baking. Bake until the pastries are golden brown, crisp, and cooked through. Remove from the oven and let cool slightly before serving.
• To store: keep cooled bourekas in a sealed container or plastic zipper bag.
• To freeze unbaked bourekas: prepare, fill, and seal the pastries but do not brush with egg yolk or bake. Place the unbaked bourekas in a single layer in an airtight container or freezer bag, separating layers with parchment paper, then freeze.
• To bake from frozen: arrange frozen bourekas on a baking sheet sprayed with nonstick cooking oil spray, brush with the whisked egg yolk, sprinkle seeds if desired, and bake at 350°F for 35–45 minutes, until golden brown, crisp, and heated through.