In a large bowl, combine the ingredients for the crust: the flour, sugar, cornmeal and salt.
Add butter and with a pastry cutter, blend it in until the mixture resembles coarse meal.
Pour the buttermilk into the bowl and stir until the dough begins to stick together.
In each galette I’ve made, the dough hasn’t stuck together, so I just gather it into a ball as much as possible with my hands and then stick the rest of the dough remnants onto it while it’s sitting on the plastic wrap. (If you do have a problem with it consolidating, add one and a half teaspoon more of buttermilk.)
Remove the dough ball from the bowl and adhere any remaining pieces of dough to it, then wrap in plastic wrap.
Refrigerate for an hour.
The galette dough is able to be made in advanced and can be refrigerated up to three days before being used.
When ready to bake, preheat the oven to 350 degrees F.
In a medium bowl, whisk together the sugar, tapioca, cardamom, and orange zest.
Add the strawberries and rhubarb and toss to completely coat. Let sit for 15 minutes before placing onto the crust.
Place a piece of parchment on a baking sheet or pizza stone.
Remove the dough from the refrigerator and roll it out, starting from the center, into a circle (or as close as you can get it) 14 inches across.
Fold the edges of the dough up over the filling, overlapping the dough if necessary, and gently pressing the creases to ensure that creases are sealed.
Prepare the egg wash by combining the beaten egg with the teaspoon of heavy cream. (Using heavy cream will help the crust brown where as mixing it with water will give it a more shiny finish.)
Lightly brush the edge of the dough with the egg wash, and sprinkle with the turbinado sugar.
Bake for 1 hour, until the crust is golden brown and the filling is bubbly.
Transfer baking sheet to a wire rack and allow to cool for 20 minutes before serving.
Serve warm or at room temperature.
The filling of the galette may seep through the cracks of the crust, so I roll the parchment up around the edges to prevent it from dripping into the oven when I don't use a rimmed baking sheet.