Start by peeling and cubing the russet potatoes. Boil them in salted water until fork-tender, about 15-20 minutes. Drain and let cool slightly. If using mashed potatoes, skip this step.
In a mixing bowl, mash the cooked potatoes until smooth. Add the crumbled feta, shredded kashkaval (or additional feta), and one large egg. Season with salt and pepper. Mix thoroughly until well combined and creamy.
On a lightly floured surface, roll out the puff pastry sheets to a thickness of about 1/8 inch. If using filo dough, follow the provided directions to layer and prepare the dough.
Cut the rolled-out pastry into squares or circles, about 4-5 inches across. You can use a sharp knife or a pastry cutter for clean edges.
Place a generous tablespoon of the potato cheese filling in the center of each pastry piece. Be careful not to overfill, as this can make sealing difficult.
Fold the pastry over the filling to form a triangle or a half-moon shape. Press the edges together to seal. Use a fork to crimp the edges for a decorative touch and to ensure they are tightly sealed.
Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper and spray it lightly with nonstick cooking oil spray. Arrange the bourekas on the sheet, leaving space between each one.
In a small bowl, whisk the egg yolk with a tablespoon of water. Brush this mixture over the tops of the bourekas for a beautiful golden color as they bake. If desired, sprinkle sesame or poppy seeds on top for added flavor.
Bake the bourekas in the preheated oven for 20-25 minutes or until they are golden brown and puffed. Keep an eye on them towards the end to ensure they don’t over-brown.
Once out of the oven, let the bourekas cool for a few minutes before serving. These are best enjoyed warm, but they also make a great snack at room temperature!
