Step 1

Put oven rack in middle position and preheat oven to 425°F. Line a large baking sheet with foil and butter foil.

Step 2

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.

Step 3

Stir together blueberries, cornstarch, zest, juice, cinnamon, salt, and 1/2 cup sugar in a large bowl until combined.

Step 4

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.

Step 5

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.

Step 6

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.