In a large skillet, melt a tablespoon of butter over medium heat. Add thinly sliced onions and cook, stirring occasionally, for about 20–30 minutes until golden brown.

Add minced garlic and fresh mushrooms to the skillet. Cook for an additional 5–7 minutes, stirring frequently, until the mushrooms are tender.

Preheat your oven according to the puff pastry package. Roll out the thawed puff pastry on a baking sheet lined with parchment paper.

Spread the caramelized onions evenly over the prepared puff pastry, then layer the sautéed mushrooms on top. Sprinkle grated Gruyere cheese over everything.

Bake for 20–25 minutes, or until the pastry is puffed and golden brown.

Let the tart cool slightly before slicing, then serve warm or at room temperature.