Thaw puff pastry in the fridge until you're ready to use it.
Sautée chopped onion and kale in a pan over medium heat until wilted.
Add to a medium bowl with the mashed potatoes.
Stir in crumbled feta and one egg. Mix well and season with salt and pepper. Set aside.
Line a baking sheet with parchment paper.
On a lightly floured surface, roll out the pastry to a 12x12” square. Cut the pastry dough into 4” squares.
Place 2 tbsp of the potato filling in the centre of each square, spreading it out to ¼ inch from the edge.
Fold the dough diagonally to make a triangle. Pinch firmly along edge of the triangle and crimp with a fork to seal the edges.
Repeat this process for the remaining squares.
Place the bourekas on the prepared baking sheet, leaving space to give them room to expand during baking. Place in the refrigerator for 45 minutes.
Preheat oven to 425°F.
In a small bowl, whisk the egg yolk.
Use a pastry brush to brush the egg wash onto the surface of each boureka. Sprinkle with sesame seeds.
Bake for 30-35 minutes until golden brown.