In a large skillet, heat 2 tablespoons of olive oil over medium-low heat. Add thinly sliced yellow onions, along with a generous pinch of salt and some freshly ground black pepper. Cook the onions slowly, stirring frequently, for about 35-45 minutes until they are deeply caramelized, golden brown, and reduced in volume.

Once the onions are caramelized to a sweet, jam-like texture, transfer them to a bowl and let them cool completely.

While your filling cools, preheat your oven to 400°F (200°C). Roll out the thawed puff pastry on a lightly floured surface to about a 1/8-inch thickness.

Spread the cooled caramelized onion mixture evenly across the rolled-out pastry, keeping a half-inch border around the edges. Sprinkle fresh thyme over the onion layer.

Carefully roll the pastry tightly from the edge with the filling, ensuring the edges remain secure.

Using a sharp knife, cut the rolled pastry into ½-inch thick rounds and place them on a parchment-lined baking sheet.

Bake in the preheated oven for 18-22 minutes, or until they are golden brown and puffed to perfection.

Remove the pinwheels from the oven and transfer them to a serving platter. Garnish with fresh thyme sprigs.