Heat olive oil in a large sauté pan. Add onions and salt. Cook, stirring occasionally, for 25–35 minutes over medium-low heat until onions are caramelized.

Remove from heat and add balsamic vinegar.

Preheat oven to 375 degrees Fahrenheit.

On a lightly-floured work surface, roll the puff pastry out into a rectangle. Fold half an inch to one inch of the edges in toward the center on all sides. Transfer to a lined baking sheet.

Brush entire surface of the dough with duck sauce. Add the caramelized onions evenly over the top, followed by the beef fry.

Bake for 25–35 minutes, until the pastry is golden and cooked, and the toppings are brown. Allow to cool down and slice into six to eight pieces.