Caramelize Onions: In a large skillet, combine sliced yellow onions, olive oil, salt, and black pepper. Cook over medium-low heat for 35-45 minutes, stirring frequently, until golden brown and jammy.

Cool Filling: Transfer the caramelized onions to a bowl to cool completely.

Prepare Puff Pastry: On a floured surface, unroll the puff pastry and spread the cooled onion mixture evenly, leaving a half-inch border.

Roll and Cut: Sprinkle thyme over the filling, roll the pastry tightly into a log, and slice into ½-inch rounds.

Bake: Preheat oven to 400°F and arrange pinwheels on a baking sheet. Bake for 18-22 minutes until golden brown.

Serve: Cool for a few minutes, garnish with thyme sprigs, and serve warm.