Begin by combining all-purpose flour, cold unsalted butter, granulated sugar, and salt in a mixing bowl. Cut the butter into the flour mixture until it resembles coarse crumbs. Gradually add ice water, mixing until the dough holds together. Divide the dough into two discs, wrap each in plastic wrap, and refrigerate for at least 2 hours.

In a large bowl, toss the cut rhubarb pieces with granulated sugar and finely grated lemon zest. Allow this mixture to sit for at least 2 hours.

After chilling, roll out one disc of dough on a floured surface until it fits a 9-inch pie plate, then transfer into the plate.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Strain the rhubarb after maceration, keeping the juices. Combine half the juices with corn starch in a saucepan and boil until thickened; then mix in the remaining rhubarb and juices.

Pour the thickened rhubarb mixture into the chilled pie crust, and roll out the second disc to form the top crust; crimp the edges to seal.

Cut slits in the top crust for steam vents, brush with egg wash, and sprinkle with additional sugar.

Bake the pie for 1 to 1¼ hours or until bubbling and golden brown.

Let the pie cool on a wire rack for at least 2 hours before slicing.