Make pie dough and wrap in plastic wrap and store in the fridge for at least 30 minutes
To make the filling, combine the rhubarb, strawberries, sugar, tapioca starch, lemon juice, and salt in a large bowl. Stir to combine and set aside.
On a lightly floured surface, roll out the dough into a circle that's about 12 inches in diameter. Trim the edges if you'd like. Transfer the dough to the prepared baking sheet.
Spread the fruit mixture on top of the dough, leaving about a 2-inch border around the edges of the dough. Fold the edges of the dough up and over the fruit filling, crimping the dough as you go.
In a small bowl, whisk together the egg yolk and water to create an egg wash. Brush the egg wash over the top of the dough.
Sprinkle sugar over the top of the dough and fruit mixture. (you can use turbinado or regular sugar)
Bake the galette for 35-40 minutes at 375F, or until the crust is golden brown and the fruit filling is bubbly.
Let the galette cool for a few minutes before slicing it with a sharp knife. Serve the galette warm with a scoop of vanilla ice cream, if desired.
Tapioca Starch is my favorite pie thickener, but you can use tapioca pearls or corn starch too. With corn starch the filling just will be jucier.