Heat olive oil in a sauté pan over medium heat. Sauté onion until translucent. Add spices to pan and cook until toasted, around 1 minute. Add ground lamb and cook until no longer pink, breaking up into small pieces with a wooden spoon as you cook. Remove from heat and allow to cool slightly.
Combine butternut squash and lamb mixture in a medium bowl.
Preheat oven to 375°F.
Roll out each sheet of puff pastry. Cut each sheet into nine even squares. Using a rolling pin, roll out each square slightly.
Scoop 1 heaping Tbsp into the corner of each square. Fold puff pastry over filling, forming a triangle. Using the tines of a fork, crimp the edges.
Repeat with second sheet of puff pastry.
Brush each boureka with beaten egg. Top with sesame seeds, nigella seeds or poppy seeds if desired.
Bake 18-22 minutes, until golden on top.
