Preheat the oven to 400°.
Line a baking sheet pan with parchment paper.
On a floured board, roll the dough to a 10-inch circle, then put the dough circle onto the parchment-lined sheet pan. Cover the dough with plastic wrap and chill it in the refrigerator for 15 minutes.
Meanwhile, in a medium bowl whisk together the sugar, cinnamon, nutmeg, salt, and cornstarch.
Peel and slice the apples into another small bowl and mix in the lemon juice.
Remove the pie dough from the refrigerator and remove the plastic wrap. Mound the apples in the center of the circle of dough, leaving a 3-inch border of dough.
Pour the melted butter over the apples.
Gently fold the dough over the apples, pleating as you go around the circle. (If you want to, you can dampen the two pieces of dough at each pleat with water and press them to be sure they seal. I usually press the pleats hard enough so I don’t have to use water.)
Sprinkle coarse sugar over the crust.
Bake at 400° for 30-40 minutes until the crust is golden brown.
Remove the sheet pan to a wire rack and let it cool in the pan for 1 hour.
Use a large bench scraper or two metal spatulas to transfer the galette to a serving plate.
Store the galette covered in the refrigerator.