Preheat oven to 375'
In a small bowl, blend feta cheese, green onions and egg. Cut pastry into 12 (3") squares. Place a mounded tablespoon of feta mixture in the center of each square. Moisten edges with water, and fold pastry over filling to form triangle. Press edges together firmly with a fork to seal. Lightly brush pastries with the egg yolk mixture.
Bake for 20 minutes in the preheated oven, or until golden brown.
Serve warm or a room temperature.