3 tablespoons extra virgin olive oil
2 large onions, cut into thin half moons
2-3 cups sliced mushrooms
1 teaspoon salt
5 sprigs fresh thyme, leaves stripped and chopped
2-3 teaspoons balsamic vinegar
1-2 sheets frozen puff pastry, defrosted but kept cool
freshly grated cheese – keep it simple with a grated Romano or extra cheesy with a Cheddar or Gouda
Heat the olive oil in a large sauté pan over medium heat. Add the onions and stir continuously until softened, about 8 to 10 minutes. Sprinkle in the salt, add the sliced mushrooms and continue stirring until the onions release their moisture and the pan becomes more and more dry. When this happens, add half of the thyme and the balsamic vinegar and reduce the heat to medium low. Cook the veggies, stirring occasionally, until caramelized and golden, about 20 -25 minutes or so. (If the onions start to brown, reduce the heat as low as your stove goes.) Remove from the heat and set aside.
Preheat the oven to 180C. Adjust the oven rack to the center position.
Unfold the sheet/s of defrosted puff pastry on a parchment-lined baking sheet. Use 2 for a larger tart – just attach the puff pastry dough; you can just use 1 sheet if desired. Fold all the edges over to create the pretty border around your tart. Top the center of the rolled out puff pastry evenly with the onion, mushrooms, thyme mixture. Sprinkle with the chosen cheese.
Bake the tart for about 25 to 35 minutes, until the pastry is golden brown and flaky. Remove it from the oven, sprinkle the remaining thyme over the top, and let it cool for about 10 minutes before slicing and serving. (Any leftover tart can be kept at room temperature, lightly covered with parchment. Best to crisp it in an oven or toaster oven prior to serving again.