In a mixing bowl, toss the sliced pears with lemon juice, brown sugar, cinnamon, nutmeg, ginger, and cornstarch until the pears are evenly coated. Let sit for about 10 minutes to release some juices.
On a lightly floured surface, roll out the pie crust into a 12-inch round. If using store-bought, you can lay it directly on baking parchment.
Place the pear mixture in the center of the rolled-out crust, leaving about a 2-inch border around the edges.
Gently fold the edges of the dough over the pears, pinching the dough together at intervals to create a rustic edge.
In a small bowl, whisk together the egg and milk. Brush the egg wash over the folded edges of the galette.
Sprinkle the coarse sugar over the crust for added sweetness and texture. Bake in the preheated oven for about 25-30 minutes, or until the crust is golden and the pears are tender.
Allow the galette to cool on a wire rack for at least 10 minutes before slicing.
Notes
Serve warm or at room temperature, topped with vanilla ice cream or whipped cream.