Preheat the oven to 400°F (200°C). Roll out the puff pastry and fit it into a pie dish. Trim any excess pastry and prick the bottom with a fork. Bake for 10 minutes until lightly golden.

In a skillet, heat olive oil over medium heat. Add sliced onions, salt, and sugar. Cook, stirring occasionally, for about 30 minutes until the onions are golden brown and caramelized.

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

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

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