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. 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 degree F oven until the crust is golden and the apples are tender, about 30-40 minutes. 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 slices. Top each slice with vanilla ice cream or whipped cream, if desired.