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 olive oil over medium heat. Add sliced onions, salt, and sugar. Cook, stirring occasionally, until onions are golden brown and caramelized, about 25-30 minutes.

In a mixing bowl, whisk together the heavy cream, eggs, thyme, black pepper, and half of the gruyère cheese.

Spread the caramelized onions evenly over the pastry crust. Pour the cream mixture over the onions and sprinkle with the remaining gruyère cheese.

Bake in the preheated oven for 25-30 minutes or until the filling is set and the top is golden brown.

Let the tart cool for a few minutes before slicing. Serve warm or at room temperature.