Thaw puff pastry at room temperature for 20 to 30 minutes. Unfold. If pastry does not unfold easily, thaw for an additional 5 to 10 minutes and retry.
Preheat oven to 400 degrees F.
On a lightly floured surface, roll out pastry to a 10 by 12-inch rectangle. Transfer to an ungreased baking sheet. With a dry pastry brush, brush away any excess flour.
In a small bowl, lightly beat together egg and 1 tablespoon water to create an egg wash.
With the pastry brush, brush entirety of dough sheet surface with egg wash. Along all 4 sides, fold over a 1/2-inch of the edge to create a ridged border around pastry. Brush top of this rim with egg wash as well. Prick pastry (inside the rim) all over with a fork (this ensures that the dough inside the border does not puff up like the outer rim will).
Arrange thinly sliced pears on pastry, covering all of surface area, but in a single layer. Sprinkle pears with sugar, pear liqueur, and dot with cubes of butter.
Bake for 15 to 20 minutes or until puffed and golden brown.
While tart is baking, combine peach preserves and peach nectar in a microwave-safe bowl. Stir to combine. Heat on high for 15 seconds at a time until preserves have melted, stirring each time. Once mixture has completely melted, push through a fine mesh strainer to remove any lumps or pieces of fruit. Set aside.
Once tart has finished baking, remove from oven. Brush peach glaze over entire tart, both fruit and pastry. Cool to room temperature and serve.