Peel, core, and slice the apples, tossing them in lemon juice to keep them from discolouring.
Add all but ½ teaspoon of the sugar and all of the cinnamon to the apples and stir. Set aside while you prepare the crust.
Dust parchment paper and rolling pin with white rice flour. Roll pie dough into a flat, round shape about 13-inches (33 cm) in diameter on the sheet of dusted parchment paper.
Arrange the apple filling into the centre of the crust, leaving 2 - 3 inches (5 - 7 cm) around the edge for folding.
Fold in the edges of the crust all the way around the galette, leaving some filling exposed in the centre.
Brush some beaten egg over the top of the pastry. Sprinkle it with reserved raw sugar.
Slide the whole thing, parchment and all, onto a baking sheet. Bake at 425°F (218°C) for 10 minutes.
Reduce the heat to 350°F (177°C), and continue to bake for another 25 or 30 minutes, until the apples are soft when poked with a knife. If your galette is browning too quickly, cover it with foil for the last 10 - 15 minutes of baking.
Lemon juice helps to keep the sliced apples from turning brown.
Letting the apple slices to sit with sugar for a few minutes allows the sugar to be dissolved into the apple juices.