HANDS-ON 20 minutes
TOTAL 1 hour, 10 minutes
Preheat the oven to 400°F.
Peel and core the pears; cut into quarters. Set aside.
Stir together the sugar, lemon zest, lemon juice, and ginger in a small bowl. Transfer the mixture to a 10-inch nonstick ovenproof skillet or tarte tatin pan. Cook over medium-low, stirring constantly, until the sugar dissolves, about 8 minutes. Increase the heat to medium. Cook until the sugar begins to caramelize and turns golden brown, about 7 to 10 minutes.
Arrange the pear slices over the sugar mixture, slightly overlapping the slices and filling the skillet. Scatter the butter pieces and thyme leaves over the pears. Cook over medium until the pears are crisp-tender, about 10 minutes. Remove the skillet from the heat; let stand while preparing the pastry.
Unroll the puff pastry sheet on a lightly floured surface, and cut out an 11-inch circle using a small knife. Place the pastry round over the caramelized pears, tucking the extra pastry beneath them.
Bake in the lower third of the oven until the pastry is golden brown,
30 to 35 minutes. Remove from the oven, and cool 10 minutes.
Place a large plate or cutting board on top of the skillet, and invert the tart. Garnish with the thyme leaves. Serve warm.