1. Make the Crust:

   • In a large bowl, whisk together 2½ cups all-purpose flour, 1 tsp salt, and 1 tsp granulated sugar.

   • Add 1 cup cold, cubed unsalted butter and cut it into the flour using a pastry cutter or your fingers until the mixture resembles coarse crumbs with pea-sized butter pieces.

   • Add 6–8 tbsp ice water, one tablespoon at a time, mixing until the dough just comes together (it should hold when squeezed but not be sticky).

   • Divide the dough into two equal portions, shape into disks, wrap in plastic wrap, and chill in the refrigerator for 1 hour.

2. Prepare the Oven and Pan:

   • Preheat the oven to 375°F (190°C).

   • Lightly grease a 9×13-inch baking pan or line with parchment paper for easier removal.

3. Make the Filling:

   • In a large bowl, combine 3 cups chopped rhubarb, 2 cups fresh raspberries, 1 cup granulated sugar, ¼ cup cornstarch, 1 tbsp lemon juice, and 1 tsp vanilla extract.

   • Stir gently to mix, being careful not to crush the raspberries. Set aside.

4. Roll Out the Bottom Crust:

   • On a lightly floured surface, roll out one dough disk into a rectangle slightly larger than the 9×13-inch pan (about 11×15 inches).

   • Transfer the dough to the prepared pan, pressing it into the corners and allowing excess to hang over the edges.

5. Add the Filling:

   • Pour the rhubarb-raspberry filling into the crust and spread evenly.

6. Add the Top Crust:

   • Roll out the second dough disk into a similar-sized rectangle.

   • For a solid top, lay it over the filling, trim excess, and crimp the edges to seal. Cut a few slits for steam to escape.

   • For a lattice top, cut the dough into 1-inch strips. Lay half the strips over the filling in one direction, then weave the remaining strips perpendicularly to create a lattice. Trim excess and crimp edges.

7. Finish the Crust:

   • Beat 1 egg and brush it lightly over the top crust for a golden finish.

   • Sprinkle with 1 tbsp coarse sugar, if using, for a sparkly crunch.

8. Bake:

   • Bake for 45–50 minutes, or until the crust is golden brown and the filling is bubbling.

   • If the edges brown too quickly, cover them with foil halfway through baking.

9. Cool and Serve:

   • Let the pie cool completely in the pan (about 2 hours) to allow the filling to set.

   • Slice into 12 squares and serve at room temperature or slightly warmed.

Cooking Tips

• Cold Butter: Keep butter cold to ensure a flaky crust; chill the dough if it softens during rolling.

• Gentle Mixing: Stir the filling gently to keep raspberries intact for a beautiful presentation.

• Prevent Soggy Crust: Ensure the filling is thickened with cornstarch, and let the pie cool fully to set the juices.

• Lattice Tips: If weaving a lattice, work quickly to keep the dough cold, and use a ruler for even strips