Put oven rack in middle position and preheat oven to 425°F. Line a large baking sheet with foil and butter foil. Transfer ice cream to a microwave-safe bowl and microwave at 30 percent power at 10-second intervals until softened, about 50 seconds total. Stir in zest and juice, then thinly spread in a shallow baking pan and freeze while making galette. Stir together blueberries, cornstarch, zest, juice, cinnamon, salt, and 1/2 cup sugar in a large bowl until combined. Unwrap pie dough and unfold onto baking sheet, then spoon blueberry mixture onto center of dough, leaving a 1 1/2-inch border around edge. Fold edge of dough over 1 inch of blueberry mixture, pleating dough, then dot blueberry filling with butter pieces. Lightly brush pastry with some of beaten egg and sprinkle with remaining teaspoon sugar. Bake until blueberry filling is bubbling and pastry is golden, 25 to 30 minutes. Cool slightly on baking sheet on a rack. Serve warm galette with scoops of lemon ice cream.