Line a rimmed baking sheet with parchment or a silicone mat and spray with cooking spray. Set aside. Remove the dough from the refrigerator and let stand for 15 minutes. Place the dough between 2 sheets of lightly floured waxed paper and roll out into an 11-inch circle. Brush off the excess flour. Transfer the dough to the prepared baking sheet, cover with plastic wrap and refrigerate for 15-30 minutes.
Meanwhile, preheat oven to 400°F. In a large bowl, stir together the apples, sugar, lemon juice, cinnamon, nutmeg, salt and cornstarch.
Arrange the apple filling in the center of the dough, mounding the fruit slightly and leaving a 2-inch border. It will look like you have a lot of apples piled high in the center of the galette before baking, but don't worry! The apples cook down quite a bit in the oven. Gently fold the edges of the dough over the apples, pleating loosely. Brush the dough with the beaten egg (you won’t need all of the egg). Lightly sprinkle sugar over the apples and on the crust.
Bake the galette on the middle rack in the 400°F oven until the crust is golden and the apples are tender, about 30-40 minutes. Start checking the galette after about 28 minutes in the oven (to make sure that the crust doesn't burn), but it will likely require closer to 40 minutes to really achieve a rich brown crust and tender apples. Transfer the pan to a wire rack and let the galette cool completely, about 1 hour.
Warm the jam in a small saucepan over low heat until it’s a nice, thin consistency. Brush the apples with the jam.
Cut the galette into 6-8 triangles. The galette slices most easily if you allow it too cool for about an hour before serving. Top each wedge with whipped cream or a scoop of vanilla ice cream. A drizzle of caramel sauce would also be delicious.