The Method
1. Craft the Perfect Crust
In a large mixing bowl, whisk together your flour, sugar, and salt. Toss in those cold cubes of butter. Using a pastry cutter—or honestly, your fingertips—work the butter into the flour. You aren't looking for a smooth dough yet; you want "shaggy" crumbs with pea-sized chunks of butter still visible. That’s the secret to a flaky crust. Drizzle in the ice water a tablespoon at a time, mixing until it just barely holds together. Shape it into a flat disk, wrap it tight, and let it chill in the fridge for at least an hour. Do not skip the chill time; it’s what prevents the crust from shrinking in the oven.
2. Prepare the Fruit
While your dough rests, prep the filling. In a large bowl, toss your rhubarb and raspberries with the sugar, cornstarch, vanilla, and lemon juice. The sugar will immediately start to draw the juices out of the fruit, creating a thick, ruby-red syrup. Let this sit for a few minutes so the cornstarch can really latch onto those juices.
3. Assemble with "Rustic" Flair
Preheat your oven to 375°F (190°C). On a lightly floured surface, roll your chilled dough out into a rough 12-inch circle. Don't worry if the edges are frayed or uneven—that’s the "rustic" part! Transfer the dough to a baking sheet lined with parchment paper. Mound the fruit mixture right into the center, leaving about a 2-inch border of dough all the way around.
4. The Fold and Bake
Gently fold the edges of the dough up over the fruit, pleating and overlapping it as you go. The center should remain open. Brush the exposed dough with your egg wash and sprinkle generously with the coarse sugar. Bake for 40 to 45 minutes. You’ll know it’s ready when the crust is a deep golden brown and the fruit filling is bubbling and thick.