1. Make the Dough: In a large bowl, whisk together buckwheat flour, all-purpose flour (if using), sugar, and salt. Cut in cold butter until pea-sized pieces remain. Gradually add ice-cold water just until the dough comes together. Form the dough into a disk, wrap it in plastic, and refrigerate for 30 minutes.

2. Prepare the Filling: In a separate bowl, combine raspberries, rhubarb, sugar, and cornstarch. Stir gently to coat the fruit. Add lemon zest if desired for extra brightness.

3. Roll the Dough: Preheat the oven to 400°F (205°C). On a lightly floured surface, roll out the chilled dough into a 12- to 14-inch circle (rustic edges are fine).

4. Assemble the Galette: Transfer the rolled dough to a parchment-lined baking sheet. Spoon the fruit filling into the center, leaving a 2-inch border. Fold the edges of the dough over the filling, pleating as needed. Brush the crust with egg wash (or milk) and sprinkle with coarse sugar.

5. Bake: Place the galette in the preheated oven and bake for 30–40 minutes, or until the crust is golden brown and the filling is bubbling. Let it cool slightly before slicing.