To make the dough, combine flour, sugar, cornmeal, and salt in a processor; pulse 2-3 times. Add butter and pulse 4-5 times, or until mixture resembles coarse meal. With the processor running, slowing pour the buttermilk through the chute, processing until the dough forms a ball. Remove the dough ball and adhere any remaining pieces of dough to it, then wrap in plastic wrap or parchment paper. Refrigerate for at least 45 minutes before rolling out.
Center a rack in the oven and preheat to 350 degrees F.
To make the strawberry rhubarb filling, combine all ingredients in a medium bowl, and toss gently, making sure that the cornstarch is coating the fruit. Set aside.
Remove the dough from the refrigerator and place on a sheet of parchment paper. Starting at the center of the dough, roll out, forming a 14-15-inch wide circle. Place the dough with parchment paper on a baking sheet. Add the fruit mixture to the center of the dough, leaving a 2-inch border. Fold the border over the filling, overlapping where necessary and pressing gently to adhere the folds.
Lightly brush the edge of the dough with the egg and sprinkle with turbinado sugar. You don't need to use the whole egg, just enough to brush the edge of the galette.
Bake for 1 hour, or until crust is golden brown and the filling is bubbly. Transfer the baking sheet to a wire rack to cool for at least 20 minutes. Serve warm or at room temperature.
*Note-you can make the galette dough in advance. It will keep in the refrigerator for 3 days. Try serving the galette with vanilla ice cream for an extra special dessert.