Strawberries and rhubarb are a classic springtime combo, but you could use cherries or apricots if you prefer. Baking this pretty galette on a heated surface like a baking stone creates a crisp, sturdy bottom.
Line a baking sheet with parchment paper.
Roll dough on a lightly floured surface into a 12-inch round about 1/8 inch thick. Transfer to the prepared pan.
Stir granulated sugar, crystallized ginger, fresh ginger and nutmeg (or pepper) in a large bowl. Add rhubarb and strawberries. Toss to coat. Sprinkle with cornstarch and toss to coat.
Distribute breadcrumbs (or semolina) over the center of the dough, leaving a 2-inch border. Evenly spread the filling over the breadcrumbs (or semolina). Lift the edge of the dough, folding it over and partly covering the filling and making slightly overlapping pleats as you work your way around. Gently press on the top and sides of the dough to ensure a good seal. Dot the exposed filling with butter. Whisk egg and water in a small bowl and brush over the dough; sprinkle with turbinado sugar. Chill the galette for 30 minutes.
About 20 minutes before baking, place a baking stone or inverted baking sheet on the center rack of oven; preheat to 400 degrees F.
Transfer the galette on the parchment to the heated surface. Bake until the filling is bubbling and the crust has browned, 40 to 45 minutes.
Let cool on a wire rack for at least 45 minutes before serving.
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