Preheat the oven to 375°F. Line a tray with baking paper.

In a saute pan, heat 1 tablespoon oil over medium heat. Add the onions, cooking for 15-20 minutes or until golden.

Add the onions to a medium bowl with the mashed potatoes, pepper, and sea salt. Set aside.

On a lightly floured surface, unfold the puff pastry. Slice each piece of the dough into 6 squares.

Place about 1 tablespoon in the center of each square.

Fold the corners in to meet on the top of the filling, twisting to seal into a round shaped knish.

Repeat until there is no more dough.

Transfer to the lined baking tray.

Brush with whisked egg.

Bake for 20-25 minutes, or until golden.