In a large saucepan of salted boiling water, blanch the spinach for 30 seconds. Drain and rinse under cool water. Squeeze out as much water as possible from the spinach. Chop the spinach and transfer it to a medium bowl. Add the onion, feta, allspice, olive oil and lemon juice, season with salt and pepper and mix well.

Preheat the oven to 425°. Line a baking sheet with parchment paper. On a lightly floured work surface, using a lightly floured rolling pin, roll out the puff pastry 1/8 inch thick. Using a 3 1/2-inch round biscuit cutter, cut out 12 rounds; transfer them to the prepared baking sheet. Working with one round at a time, brush the edge with some of the beaten egg, then mound about 1 tablespoon of the spinach filling in the center. Bring three sides of the dough together in the center to form a triangle and pinch to seal. Repeat with the remaining pastry rounds, egg wash and spinach filling.

Brush the pastries all over with the beaten egg and sprinkle with the sesame seeds. Bake for about 20 minutes, until the pastry is puffed and golden. Serve hot.