Preheat the oven to 400°F. Line a large sheet pan with parchment paper.

Let the refrigerated pie crust sit at room temperature for about 10 minutes, or until it is pliable enough to unroll without cracking.

Unroll the pie crust onto a lightly floured work surface. Use a rolling pin to gently roll it into a circle measuring approximately 12 inches across.

Transfer the crust to the prepared sheet pan.

Add the blueberries, granulated sugar, lemon juice, and cornstarch to a large bowl. Gently toss until the blueberries are evenly coated and no dry cornstarch remains.

Spoon the blueberry filling into the center of the crust, leaving a 2-inch border around the outside.

Working your way around the galette, gently lift the edge of the crust and fold it toward the filling. Slightly overlap the dough with each fold to create a rustic border. Leave the center of the blueberry filling exposed.

Whisk the egg and water together in a small bowl. Lightly brush the egg wash over the folded crust. Sprinkle with coarse or granulated sugar, if desired.

Bake for 30 to 35 minutes, or until the crust is deeply golden brown and the blueberry filling is bubbling in the center.

Let the galette cool on the sheet pan for at least 15 minutes before slicing and serving. The filling will continue to thicken as it cools.