Combine the flour, salt, sugar, and butter cubes in a food processor. Pulse to combine. Add 5 tablespoons of ice cold water. Pulse until the dough comes together. It should hold together on its own but still be slightly crumbly. You may need to add additional tablespoons of water. Add one at a time to avoid adding too much.
Roll the dough into a ball then flatten into a disk with your palm. Wrap tightly in plastic wrap. Refrigerate for at least 30 minutes or overnight.
Preheat the oven to 375 degrees F/190 degrees C.
Prepare the strawberry rhubarb filling. Combine the diced rhubarb, sliced strawberries, lemon zest, granulated sugar, vanilla extract, and cornstarch in a large bowl. Stir well. Let the filling sit and macerate, stirring occasionally, while you prepare the dough for it.
Place a piece of parchment paper large enough to cover a large baking sheet (preferably rimless) on the counter. Sprinkle it with flour.
Remove the galette dough from the fridge and unwrap it. Put the dough disc on the parchment paper. Use a rolling pin dusted with flour to roll it out to a 14-inch circle. Work quickly to prevent the dough from getting too warm.
Carefully slide the parchment paper onto the large baking sheet (preferably rimless). You can also roll the dough on the parchment on top of the baking sheet if you are worried about transferring it.
Use a slotted spoon to transfer the filling to the center of the dough circle. You don’t want to transfer too much of the maceration juices in the bowl or else the galette may be soggy. Spread the filling out while leaving about 2 inches of a border. The galette will seem very full, but the filling cooks down in the oven.
Fold the remaining border of dough over the filling, pinching it as needed to overlap itself a bit.
Brush a thin layer of the beaten egg over the crust. You won’t use it all. Sprinkle the crust with turbinado sugar.
Bake in the preheated oven for 30 to 40 minutes or until the filling is bubbling and the edges are lightly browned.
Remove from the oven and let cool for 5 to 10 minutes before slicing into pieces. Serve warm with a scoop of vanilla ice cream if desired!
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