1. Preheat oven to 375°F (190°C).
2. In a skillet, heat olive oil over medium heat. Add onion and garlic, and sauté until translucent.
3. Add chopped spinach to the skillet and cook until wilted. Season with salt and pepper. Remove from heat and let cool slightly.
4. Roll out puff pastry on a floured surface and cut into squares (approximately 4×4 inches).
5. In a bowl, mix the cooked spinach with feta cheese.
6. Place a spoonful of the spinach mixture in the center of each pastry square.
7. Fold the pastry over to form triangles, sealing the edges with a fork.
8. Brush the tops with beaten egg and sprinkle with sesame seeds if desired.
9. Bake for 20-25 minutes, or until the pastries are golden brown.

Enjoy!