Begin by slicing the onions into thin rings. In a large skillet, heat a tablespoon of olive oil over medium heat, then add the onions and a splash of sherry. Cook the onions for about 30 minutes, stirring frequently, until they turn a rich golden brown and caramelized in color, releasing a sweet aroma.

Once the onions are caramelized, add the sliced mushrooms, minced garlic, and a sprinkle of fresh thyme to the skillet. Sauté everything together for 5-7 minutes, until the mushrooms are tender and have absorbed the savory flavors. Remove from heat and let the filling cool slightly before assembling the tart.

Preheat your oven to 400°F (200°C). Prepare a baking sheet lined with parchment paper.

Roll out the thawed puff pastry on a lightly floured surface and transfer it to the prepared baking sheet. Layer the caramelized onions evenly across the base, followed by the sautéed mushrooms. Finally, sprinkle a generous amount of Gruyere cheese on top.

Carefully place the assembled tart into the preheated oven. Bake for approximately 20-25 minutes, or until the pastry is golden brown and puffed up, and the cheese is bubbling.

Once baked, remove the tart from the oven and let it cool for a few minutes. Cut into wedges and serve warm.