Festive Apple Galette

To prepare crust: Combine whole-wheat pastry flour, all-purpose flour, cornmeal, granulated sugar and salt in a medium bowl. Using your fingers, crumble butter into the flour mixture until you get a coarse sandy texture. Combine yogurt with ice water in a small bowl. Using a fork, incorporate the yogurt mixture into the flour mixture a little bit at a time. Add just enough to moisten the dough. Pat the dough into a disk and wrap in plastic wrap; chill in the refrigerator for at least 2 hours before rolling. To prepare filling: Just before you roll out the dough, toss apples, lemon juice and brown sugar (enough to sweeten the filling to your taste) in a large bowl until combined. Preheat oven to 425ºF. Line a large baking sheet with parchment paper or a nonstick baking mat. On a lightly floured surface, roll the chilled dough into a large circle, about 13 inches in diameter. Drape the dough over the rolling pin and carefully transfer it to the prepared baking sheet; patch any holes with your fingers. Arrange the apples in a mound in the center of the dough, leaving a 2- to 3-inch border of dough around the edge. Fold the border up and over the filling, pleating as necessary; the apples will not be completely covered. Try not to make any holes, but if you do, patch as you go. Lightly brush the tart rim with egg white and sprinkle with granulated sugar. Bake the tart for 15 minutes. Reduce the oven temperature to 350º and continue baking until the crust is nicely browned and the apples are bubbling, 30 to 40 minutes more. Carefully transfer to a wire rack to cool. Serve warm or at room temperature, cut into wedges.