Preheat oven to 400° F / 205° C. Line a half-sheet rimmed baking tray with parchment.

In a medium bowl, combine blueberries, granulated sugar, cornstarch, lemon juice, lemon zest, and ginger. Set aside.

Unroll the shortcrust on a silicone baking mat or lightly floured countertop.

Use a knife to trace around a large dinner plate or pastry ring, cutting a disc approximately 10 inches in diameter. If necessary, use a rolling pin to roll the pastry until it is wide enough to trace around the plate.

Transfer to prepared tray.

Spoon the blueberry mixture into the middle of the pastry, allowing a 1 1/2-inch border around the edge.

Fold the edges of the pastry inward to seal in the blueberry mixture.

Brush the pastry with egg wash.

Sprinkle entire galette with a generous amount of Demerara sugar.

Bake in preheated oven for 30-35 minutes, or until pastry is flaky and deeply golden brown, with blueberry juice bubbling around the edges.

Garnish with fresh thyme.

Slice into wedges. Serve warm, accompanied by vanilla ice cream.

Wrap leftover galette in foil. Store in the refrigerator for up to three days.