Preheat the oven to 400°F (200°C). Roll out the puff pastry and fit it into a pie dish. Prick the bottom with a fork and set aside.

In a skillet, heat the olive oil over medium heat. Add the sliced red onions and cook, stirring occasionally, for about 15-20 minutes until they are soft and caramelized. Stir in balsamic vinegar if using.

In a bowl, whisk together the eggs, heavy cream, salt, and black pepper. Stir in the caramelized onions and half of the gruyere cheese.

Pour the onion mixture into the prepared pastry crust. Sprinkle the remaining gruyere cheese on top. Bake in the preheated oven for 25-30 minutes until the filling is set and the top is golden brown.

Let the tart cool slightly before slicing. Serve warm or at room temperature.