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

Preheat the oven to 375°F (190°C). Roll out the puff pastry and fit it into a pie dish. Trim excess pastry.

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

Spread the caramelized onions evenly over the pastry, then pour the cream mixture on top. Sprinkle with remaining cheese.

Bake in the preheated oven for 30-35 minutes or until the filling is set and the top is golden brown.

Allow to cool slightly before slicing and serving.