Prepare the pastry and place one portion of dough onto a sheet of parchment paper dusted with flour. Layer a sheet of plastic wrap over the dough and roll (over the plastic) from the centre in all directions until approximately 12 inches (30 cm) in diameter and about ⅛ inch (3 mm) thick.

Drape the dough onto your rolling pin and transfer to the pie plate, being mindful not to pull or stretch the dough. Gently press the dough into the sides of the pie plate and trim the edges. Cover with plastic wrap and refrigerate for at least 40 minutes.

Place the remaining portion of dough onto a sheet of parchment paper dusted with flour. Place a sheet of plastic wrap on top of the dough and roll (over the plastic) from the centre toward the pastry’s edge in all directions, making a circle approximately 10” inches (27 cm) in diameter and ¼ inch (3 mm) thick. Remove the plastic and cut this into 6 (or 8) strips 2 inches (5 cm) wide. Transfer to a tray or plate with the parchment beneath, wrap in plastic and refrigerate for at least 40 minutes.

In a large bowl, combine the sliced rhubarb, 1 ½ cups (375 mL) granulated sugar, ginger, tapioca and salt in a large bowl and toss to combine. Set aside for 20 minutes to soften the tapioca.

Retrieve the dough-lined pie plate from the fridge. Scatter the walnuts along the base, if using. Tip the fruit/sugar mixture into the plate and mound the fruit in the centre. Top with any remaining sugar/tapioca mixture left in the bowl. Gently tap the pie plate on your work surface to distribute the fruit and sugar. Dot with butter.

Place three (or four) 2-inch (5 cm) strips of pastry loosely across the rhubarb in one direction, leaving a small gap between each strip. Do not press the dough in place. To weave in the first pastry strip, gently fold back 2 alternating strips to the pastry’s edge and lay a perpendicular strip of pastry over the remaining strips. Fold the strips back in place and you’ll see you’ve created your first weave. Repeat the process with the remaining 2 strips.

Trim the edges and carefully press the lattice into the pastry base with your fingers. Brush the lattice with beaten egg and dust with coarse raw or granulated sugar. Cover loosely with plastic wrap and refrigerate or freeze for about 20 minutes to firm the lattice.

Preheat a foil-or parchment-lined baking tray in a 425°F (220°C) oven.

Bake the pie on the preheated tray for 15 minutes, reduce the temperature to 375°F (190°C) and continue to bake for another 30–40 minutes, or until the pastry is golden and the rhubarb is tender. Rotate the baking tray once during baking and tent with foil as necessary to prevent burning.

Cool on a wire rack. Allow the pie to cool for at least 3 hours at room temperature before serving.