Purchased puff pastry and a half an hour or so of caramelizing onions makes this an easy appetizer to pull together. It can be done ahead and slightly reheated in the oven, but it takes just fine at room temperature, too. The ingredients are simple but the combination of crisp pastry, softly sweet onions and a hint of cheese and thyme is satisfying.
Caramelized Onion Tart
1 package puff pastry
4 large yellow onions, peeled, halved and cut into slices
3 Tablespoons olive oil
1 teaspoon salt
1/2 teaspoon freshly ground pepper
1/2 cup grated Parmesan cheese (or Romano or Asiago)
1 Tablespoon fresh thyme leaves, or 1 teaspoon dried thyme leaves
Preheat oven to 425 degrees. Roll out the pastry (or cut the pre-rolled sheets) into two strips, each about 15 inches long and 5 inches wide. Place each strip onto a parchment paper lined baking sheet. Pre-bake the strips for 10 minutes at 425 degrees. The pastry will puff up dramatically, then fall.
Heat the olive oil over medium heat in a large skillet. Add the onions and gently stir and cook them down. They shouldn't brown, but will slowly soften and caramelize. When they are completely softened and limp, sprinkle with the salt and pepper. Spoon the onions onto the pastry - if it's still puffy, just poke it with a fork and pile on the onions.
Sprinkle with the cheese and thyme leaves. Bake 10 minutes longer. Cut into crosswise pieces about 2 inches wide to serve. Let cool slightly before serving.