Combine whole wheat flour, all-purpose flour, and salt in a large bowl, then cut in the cold, cubed butter until the mixture resembles coarse crumbs.

Gradually add ice water, mixing until the dough just comes together, then form it into a disk, wrap it in plastic, and refrigerate for at least 30 minutes.

In a separate bowl, gently toss the chopped rhubarb with granulated sugar and set aside to macerate.

Preheat your oven to 375°F (190°C) and roll out the chilled dough on a lightly floured surface into a roughly 12-inch circle.

Arrange the rhubarb mixture in the center of the dough, leaving a 2-inch border, then fold the edges of the dough over the filling to create a rustic crust.

Transfer the galette to a baking sheet and bake for 40-45 minutes, or until the crust is golden brown and the rhubarb is tender.