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 the onions are golden brown and caramelized, about 25-30 minutes.

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

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

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