Make the Dough:

• In a large mixing 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 in using a pastry cutter, two forks, or your fingers until the mixture resembles coarse crumbs with pea-sized butter pieces, about 2–3 minutes.

• Add 6–8 tbsp ice water, one tablespoon at a time, mixing with a fork until the dough begins to hold together when pinched (avoid overmixing).

• Divide the dough into two equal portions, shape into flat disks, wrap in plastic wrap, and refrigerate for at least 1 hour (or up to 2 days).

• Tip: Keep the butter cold to ensure a flaky crust, and add water sparingly to avoid a tough dough.

Preheat and Prepare:

• Preheat the oven to 375°F (190°C).

• Lightly grease a 9×13-inch baking pan (or quarter sheet pan) with butter or cooking spray to prevent sticking.

• Tip: Use a metal or glass pan for even baking, and ensure it has at least 1-inch sides to contain the filling.

Prepare the Filling:

• In a large mixing bowl, gently 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 carefully with a spatula to coat the fruit without breaking the raspberries, about 1 minute. Set aside.

• Tip: Chop rhubarb into ½-inch pieces for even cooking, and mix gently to keep raspberries intact.

Roll Out the Bottom Crust:

• On a lightly floured surface, roll out one dough disk into a rectangle about 11×15 inches, large enough to fit the 9×13-inch pan with some overhang.

• Transfer the dough to the prepared pan by rolling it loosely around the rolling pin, then unrolling it into the pan.

• Press the dough into the corners and up the sides, letting excess hang over the edges.

• Tip: Work quickly to keep the dough cold, and patch any tears with extra dough, pressing to seal.

Add the Filling:

• Pour the rhubarb-raspberry filling into the crust, spreading evenly to ensure consistent thickness.

• Include any sugary juices from the bowl for flavor.

• Tip: Distribute the filling evenly to avoid thin spots, and gently shake the pan to settle the fruit.

Roll Out the Top Crust:

• Roll out the second dough disk into a 10×14-inch rectangle for a full top crust, or slightly larger if making a lattice.

• For a full crust: Lay the dough over the filling, trim excess to match the bottom crust, and crimp the edges by pressing with a fork or pinching to seal.

• For a lattice: Cut the dough into ¾-inch-wide strips (about 10–12 strips). Lay half the strips vertically across the pie, spacing evenly, then weave the remaining strips horizontally to create a lattice pattern. Trim excess and crimp edges.

• Tip: Chill the top dough briefly if it softens, and use a ruler for even lattice strips.

Apply Egg Wash:

• In a small bowl, beat 1 egg and brush it lightly over the top crust (full or lattice) using a pastry brush for a golden, glossy finish.

• Sprinkle 1 tbsp coarse sugar (if using) evenly over the crust for a sparkly, crunchy topping.

• Tip: Apply a thin layer of egg wash to avoid pooling, and use coarse sugar for texture without overpowering sweetness.

Bake:

• Place the pan in the preheated oven and bake for 45–50 minutes, or until the crust is golden brown and the filling is bubbling through the lattice or edges.

• Check for doneness: the filling should be thickened and visibly bubbling to ensure the cornstarch has activated.

• Tip: Place a baking sheet under the pan to catch drips, and cover edges with foil if browning too quickly after 30 minutes.

Cool and Serve:

• Remove the pie from the oven and let it cool completely in the pan, about 2–3 hours, to set the filling for clean slicing.

• Slice into 12 rectangles (3×4 grid) using a sharp knife, wiping clean between cuts.

• Serve at room temperature or slightly warmed, optionally with vanilla ice cream, whipped cream, or a dusting of powdered sugar.

• Tip: Cool fully to avoid a runny filling, and reheat slices briefly (350°F for 5–7 minutes) if serving warm.