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 and salt, cooking until the onions are soft and caramelized, about 20-25 minutes. Stir occasionally.

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

Pour the onion mixture into the prepared pastry crust. Sprinkle the remaining cheese on top. 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.