In a food processor, combine all-purpose flour, corn meal, granulated sugar and salt. Pulse a couple of times to sift all ingredients together
Add cold butter and pulse until the mixture is crumbly resembling coarse oatmeal
In a small bowl mix together egg yolk and iced water with a fork. Pour the egg water liquid down the feed tube of the food processor while pulsing until all of the ingredients are combined
Remove dough mixture from food processor and form into a disc on a large piece of plastic wrap. Refrigerate for at least 1 hour or overnight
Roll out the dough and fill
Remove from refrigerator and remove plastic wrap. Roll out on a lightly floured surface using a rolling pin. Shape the dough to form a 12" round circle about 1/4" thick.
Place on a parchment lined baking sheet and fill with pie filling
Fold up the edges of dough in a pleated pattern
Return to refrigerator for 1 hour.
In a small bowl, mix an egg and water with a fork. Brush the edges of the galette curst with egg and water using a pastry brush
Bake
Preheat oven to 375F°
Bake for 35-40 minutes. Remove galette from oven and let rest for a few minutes before cutting and serving