Start by making the filling (this step can be done a few hours or 1-2 days ahead of time): Heat the olive oil and butter (if using) in a large skillet over medium-low heat. Add onions and sauté for 5 minutes, stirring continuously. Add the salt, pepper and sugar and sauté for a further 5-10 minutes, continuing to stir. Add the poppy seeds and cook for 3 more minutes. Set mix aside and allow to cool completely.

To assemble: Preheat oven to 375°F. Line two baking sheets with parchment paper or silicone baking mats.

On a work surface, such a large cutting board or silicone rolling mat, lay out 1 sheet of puff pastry. Using a rolling pin, gently roll out puff pastry to make it a little longer and wider. Dust work surface as needed if the dough is soft or sticky.

Fold dough in half lengthways, and then cut into three even rectangles. Unfold each rectangle.

Place ¾-1 cup filling on one side of each rectangle. On the other side of each rectangle, cut two long slits using a sharp knife or pizza cutter.

Fold the side of dough with the slits on top of the other side of dough with the filling, so the onions and poppy seeds are peaking out through the slits. Crimp the edges using a fork all the way around. If you are worried the dough isn’t sticking to itself, use a little water to seal the edges. Repeat with the remaining two rectangles, and then repeat with the other sheet of puff pastry. You should have six bourekas.

Place three bourekas onto each prepped baking sheet. Brush the top of each boureka with egg wash.

Bake for 20 minutes or until golden all over.

Serve warm or at room temperature.