Preheat oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper.
In a mixing bowl, combine feta, mozzarella, egg, and salt until well blended. Make sure to break up any large crumbles of feta with the fork.
On a lightly floured surface, unfold puff pastry sheets. With a rolling pin, roll out each sheet to a 12x12-inch square. Cut it into thirds lengthwise and crosswise to get 9 4-inch squares, for a total of 18 squares. Brush edges of each square lightly with water.
Place one heaping tablespoon of cheese filling into the center of each square and fold the dough over the filling in half diagonally to form triangular pastries. Firmly pinch the edges together to seal. With the tines of a fork, crimp the edges well.
Make the everything-bagel seasoning by combining sesame seeds, poppy seeds, dried onion and garlic flakes.
In a small bowl, beat together the egg yolk and milk. Brush the tops of the borekas with egg wash and sprinkle with everything bagel seasoning.
Bake for 30 minutes, or until the pastries are puffed and deeply golden, and the filling is cooked through. Serve warm or at room temperature.
Substitute mozzarella with kashkaval or provolone cheese.
You can use fillo dough instead of puff pastry for fewer carbs.