Core the apples and cut into slices about1/4-inch thick. In a large bowl, toss the apples, brown sugar, granulatedsugar, lemon zest, lemon juice, cornstarch, cinnamon, nutmeg, and salt. Setaside.
Preheat the oven to 400F degrees. Roll out the chilled dough onto a piece of parchment paper until you get a circle with even thickness, about 12 inches in diameter. Does not need to be precise or perfectly round. A galette is rustic on purpose.
Gently move the parchment with the dough to a baking sheet. Arrange the apple slices in the middle, leaving about 2 inches of dough. You can line them up like dominoes or just arrange in a dome. Save the liquid that remains in the bowl.
Fold the dough border over the edges of the fruit, folding creases however your wish. This does not need to be “neat”. Make sure there are not tears or holes in the crust, pressing any together with your fingers so the juices from the fruit don’t escape during baking. Carefully spoon the extra liquid from the apple mixture over the apples in the galette.
In a small bowl, beat the egg and cream, thenbrush the crust with this egg wash. Sprinkle the crust all over with the rawsugar, sprinkling some over the apples as well, if desired. Dot the apples with the remaining cubes of butter.
Bake, rotating halfway through, for 35 to 40 minutes, or until the crust is golden brown and the apple slices are softened and bubbling. If the crust appears to be browning too quickly, you may cover the galette with aluminum foil for the last 5 minutes or so. Serve warm or at room temperature.