Golden, flaky puff pastry filled with sweet caramelized onions, nutty Gruyère cheese, and a hint of fresh thyme—these elegant little tartlets are a wonderful addition to any holiday appetizer spread. The onions can be caramelized a day or two ahead, and purchased all-butter puff pastry makes preparation surprisingly simple. Serve them warm from the oven or at room temperature with a glass of sparkling cider or another festive drink.
INGREDIENTS
2 tablespoons unsalted butter
1 tablespoon olive oil
2 large yellow onions (about 1 pound), halved and thinly sliced
½ teaspoon kosher salt, or to taste
1 teaspoon fresh thyme leaves, plus extra for garnish
1 teaspoon balsamic vinegar (optional)
Freshly ground black pepper, to taste
1 sheet frozen all-butter puff pastry (about 8–9 ounces), thawed according to package instructions
¾ cup finely grated Gruyère cheese
1 large egg, beaten with 1 teaspoon water, for egg wash
Flaky sea salt, for finishing (optional)
METHOD
Caramelize the onions. Melt the butter with the olive oil in a large skillet over medium-low heat. Add the sliced onions and kosher salt, stirring to coat. Cook slowly, stirring occasionally, for 30–40 minutes, or until the onions are deeply golden, soft, and sweet. Reduce the heat if the onions begin to brown too quickly. Add a tablespoon of water occasionally if needed to loosen any browned bits from the pan.
Stir in the thyme, balsamic vinegar (if using), and a few grinds of black pepper. Cook for another minute, then remove from the heat. Allow the onions to cool completely before assembling the tartlets.
Prepare the pastry. Preheat the oven to 400°F. Line a large baking sheet with parchment paper.
On a lightly floured surface, gently unfold or roll the thawed puff pastry into a rectangle approximately 10 × 12 inches. Using a sharp knife or pizza cutter, cut the pastry into 20 rectangles, approximately 2 × 3 inches each.
Arrange the rectangles on the prepared baking sheet, leaving about an inch between them. With the tip of a sharp knife, lightly score a border about ¼ inch inside the edges of each rectangle, taking care not to cut all the way through the pastry. Prick the centers a few times with a fork, leaving the borders untouched.
Fill the tartlets. Sprinkle approximately 1½ teaspoons of grated Gruyère in the center of each pastry rectangle. Top with a small spoonful of caramelized onions, keeping the filling inside the scored borders.
Lightly brush the exposed pastry edges with the egg wash. Avoid letting the egg wash drip down the sides, which can prevent the pastry from rising properly.
Bake. Bake for 15–20 minutes, or until the pastry is puffed, deeply golden, and crisp on the bottom. Rotate the baking sheet halfway through baking for even browning.
Remove from the oven and allow the tartlets to cool for a few minutes. Garnish with additional fresh thyme and a tiny pinch of flaky sea salt, if desired.
Serve warm or at room temperature.
YIELD: 20 miniature tartlets
Cook’s Notes
For the best flavor: Take your time caramelizing the onions. The transformation from sharp and pungent to mellow, golden sweetness is what makes these little tartlets special. Don’t rush this step by turning up the heat.
Make ahead: The onions can be caramelized up to three days in advance and refrigerated in an airtight container. The tartlets can also be fully baked one day ahead, cooled, and refrigerated. Reheat on a baking sheet in a 350°F oven for 5–8 minutes, or until heated through and crisp.
For easy entertaining: Assemble the tartlets a few hours ahead and refrigerate them, covered, until ready to bake. Bake them directly from the refrigerator, adding a minute or two if necessary.
Variations: Substitute Comté or aged white cheddar for Gruyère. For a more pronounced savory flavor, spread a very thin layer of Dijon mustard over the center of each pastry rectangle before adding the cheese and onions.
