In a small bowl, whisk together the cornstarch, pectin, and kosher salt until evenly combined, about 30 seconds. Set the bowl aside on your counter — you'll use this mixture shortly.

Lightly flour a clean work surface. Remove one portion of chilled pie dough from the refrigerator and roll it out into a circle about 14 inches in diameter, turning the dough a quarter turn after each roll to keep it even. Carefully lift the dough by rolling it loosely around your rolling pin and drape it over a 9-inch pie plate. Gently press it into the bottom and sides without stretching it. Trim any large overhangs, leaving about 1 inch of extra dough. Cover loosely with plastic wrap or a kitchen towel and refrigerate while you work on the filling.Checkpoint: You are ready to move on when the crust is fitted snugly into the plate with no air pockets and is back in the refrigerator.

Remove the second portion of chilled dough. For a traditional top crust, roll it into another 14-inch circle. For a lattice design, roll it into a rectangle about 12 x 10 inches and use a knife or pastry wheel to cut it into strips approximately ¾ inch wide. Place on a parchment-lined baking sheet, cover, and refrigerate until needed.

Prepare an ice bath: Fill a large bowl with 2 to 3 cups of ice and enough cold water to cover. Set a slightly smaller bowl on top of the ice — this is where the hot filling will cool down. Set both aside.

Drain the macerated fruit (rhubarb and strawberries that have been mixed with sugar) through a fine-mesh colander or strainer set over a large bowl. Let it drain for 2 to 3 minutes. Measure out exactly 6 tablespoons of the collected liquid and set it aside in a small cup. Discard the rest of the liquid or save it for another use such as a cocktail syrup.Checkpoint: You are ready to move on when you have your 6 tablespoons of reserved liquid measured and ready.

Place a large saucepan over medium heat. Add the drained rhubarb, strawberries, vanilla bean paste, and strawberry jam. Stir everything together. Cook, stirring frequently with a wooden spoon or heatproof spatula, until the fruit begins to soften slightly and the jam melts in, about 2 minutes. The mixture should look glossy and starting to release some juice.

Sprinkle the cornstarch-pectin mixture evenly over the fruit in the saucepan. Stir continuously for about 1 minute to prevent lumps. The mixture will look a little chalky at first — that is normal and will cook out.

Pour in the reserved 6 tablespoons of fruit liquid and stir to combine. Increase the heat slightly if needed to bring the mixture to a gentle simmer — you will see small bubbles breaking on the surface. Cook, stirring constantly, for about 2 more minutes, until the filling thickens noticeably. It should coat the back of a spoon and look glossy and slightly jammy.Checkpoint: You are ready to move on when the filling has visibly thickened and holds its shape briefly on the spoon before settling.

Carefully transfer the hot filling to the bowl set over your ice bath. Stir occasionally to help it cool evenly. Let it cool to room temperature, about 30 minutes. The filling will continue to thicken slightly as it cools. Do not rush this step — adding a warm filling to the unbaked crust can cause a soggy bottom.

About 10 minutes before the filling is fully cooled, position an oven rack in the lower third of the oven and preheat the oven to 425°F (220°C).

Remove the pie shell from the refrigerator. Spoon the cooled filling into the crust, spreading it into an even layer. Remove the top crust or lattice strips from the refrigerator. If using a solid top crust, lay it over the filling and press gently at the edges to seal. If making a lattice, weave the strips over and under each other across the top of the pie. Trim any dough that extends more than ½ inch beyond the rim of the pie plate. Fold the overhanging dough under itself to create a neat edge and crimp with your fingers or a fork. If using a solid top crust, use a small knife to cut 4 to 6 small slits in the center of the crust to allow steam to escape during baking.

Place the fully assembled pie in the freezer for exactly 15 minutes. This firms up the butter in the dough, which helps the crust stay flaky and hold its shape in the oven. After 15 minutes, remove from the freezer and immediately brush the top crust evenly with the lightly beaten egg using a pastry brush. Avoid pooling egg wash in the crimped edges.Checkpoint: You are ready to move on when the crust feels firm and cold to the touch and is lightly coated with egg wash.

Place the pie on the lower oven rack and bake at 425°F (220°C) for 15 minutes. The crust should just be starting to turn golden at the edges. Reduce the oven temperature to 375°F (190°C) — do not open the door more than necessary. Continue baking for 45 to 55 minutes, or until the crust is deep golden brown all over and you can see the filling actively bubbling through the vents or lattice. If the edges begin to brown too quickly before the center is done, loosely cover the edges with strips of aluminum foil or a pie shield.Checkpoint: You are ready to move on when the crust is uniformly golden brown and the filling is visibly bubbling — not just steaming.

Remove the pie from the oven and set it on a wire rack. Allow it to cool completely at room temperature for at least 4 hours before slicing. The filling needs this time to fully set — cutting too early will result in a runny, soupy pie.