1 sheet frozen puff pastry freshly grated Parmesan or Gruyere
1. Heat the olive oil over medium heat. Add the onions and stir until softened, about 8-10 minutes. Add mushrooms and salt and continue stirring until the pan is dry. Add balsamic vinegar and thyme, reduce heat to medium low. Cook, stirring occasionally, until caramelized and golden, about 20 -25 minutes. Remove from heat and set aside.
2. Preheat the oven to 350°F. Adjust the oven rack to the center position.
3. Unfold the sheet/s of defrosted puff pastry on a parchment-lined baking sheet. Use a cookie cutter or similar to create 2” round pastries. Top the center of each 2” round puff pastry evenly with the onion, mushrooms, thyme mixture.
4. Bake for about 25 to 35 minutes, until the pastry is golden brown and flaky. When there is about 2-3 minutes left, sprinkle on the freshly grated cheese and continue baking for the remaining time. Remove it from the oven and let it cool for about 10 minutes before serving.
5. Bake the tart for about 25 to 35 minutes, until the pastry is golden brown and flaky. When there is about 2-3 minutes left sprinkle on the cheese. Remove from the oven and let it cool for about 10 minutes before serving.