01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Heat olive oil in a skillet over medium heat. Sauté onion until soft, about 3 minutes. Add garlic and cook 1 minute more.
03 - Add spinach to the pan. Cook until wilted (or warmed through if using frozen), then transfer to a colander and squeeze out excess liquid. Let cool slightly.
04 - In a bowl, combine spinach, sautéed onion and garlic, crumbled feta, egg, dill (if using), salt, and pepper. Mix well.
05 - Unroll puff pastry and cut into 6 equal rectangles.
06 - Place a generous spoonful of filling on one half of each rectangle, leaving a border. Fold over to form parcels and press edges with a fork to seal.
07 - Place pastries on prepared baking sheet. Brush tops with beaten egg yolk and sprinkle with sesame seeds if desired.
08 - Bake for 20–25 minutes, or until golden and puffed. Serve warm.