Gingered Pear Galette

2 (9-inch) refrigerated pie crusts
2 1/2 pound ripe pears, such as Bartlett, peeled, cored and sliced thin
3 tablespoons finely grated peeled gingerroot (use the side of a spoon to scrape away the peel from the ginger before grating to retain more of the root)
2 tablespoons cornstarch
1 tablespoon finely grated fresh lemon zest
1 tablespoon fresh lemon juice
1/2 teaspoon ground cinnamon
1/4 teaspoon salt
1/2 cup plus 2 teaspoons sugar, divided
2 tablespoons cold unsalted butter, cut into pieces
1 large egg, lightly beaten
Vanilla bean ice cream, optional

Place oven rack in middle position and preheat oven to 375 degrees. Allow piecrust to warm to room temperature before handling. Butter or spray 2 jellyroll pans. In a large bowl, combine pears, gingerroot, cornstarch, zest, juice, cinnamon, salt and 1/2 cup of the sugar. Unwrap pie dough and unfold one crust onto each baking sheet. Spoon half of the pear mixture onto center of each crust, leaving a 2 inch border around the edge. Fold edges of dough in toward center over pear mixture, pleating dough, and then dot pear filling with butter pieces. Lightly brush exposed surfaces of pastry with some beaten egg and sprinkle the edges with remaining 2 teaspoons sugar. Bake until pear filling is bubbling and pastry is golden, 35 to 45 minutes. Cool on baking sheet on a rack. Serve galette with scoops of vanilla bean ice cream.