Ingredients
1 tablespoon unsalted butter
2 tablespoons extra-virgin olive oil
2 onions, thinly sliced
2 tablespoons fresh thyme leaves
All-purpose flour, for dusting
1 sheet frozen puff pastry, preferably all-butter, thawed
1/4 cup oil-cured small black olives, pitted and halved lengthwise
4 to 6 anchovy fillets, preferably salt-packed, rinsed (optional)
Preparation
Preheat oven to 425 degrees. Melt butter with oil in a medium skillet over medium-high heat. Add onions and thyme, and cook, stirring occasionally, until onions are golden and soft, about 10 minutes.
On a lightly floured surface, roll out pastry to an 8 1/2-by-15-inch rectangle, trimming edges to make them straight. Place on a parchment-lined baking sheet, transfer to oven, and immediately reduce oven temperature to 400 degrees. Bake until pastry just begins to puff, about 12 minutes.
Arrange onions in a single layer over pastry, leaving a 1-inch border all around. Top with olives and anchovies (if desired). Bake 15 minutes. Reduce oven temperature to 375 degrees, and continue baking until tart is puffed and golden around the edges, about 15 minutes more (tent with foil if crust browns too quickly). Let cool slightly before cutting into small squares and serving.
