Prepare the pie crust recipe through step 5.

Peel the peaches and then cut into 1-inch chunks. You need about 6 cups of peach chunks, which is around 950g.

In a large bowl, stir the peach chunks, blueberries, granulated sugar, cornstarch, flour, cinnamon, allspice, and lemon juice together until thoroughly combined. Set filling in the refrigerator while you roll out the dough. No need to cover it.

Adjust the oven rack to the lower third position and preheat to 425°F (218°C). Place a baking sheet on the bottom oven rack to catch any bubbling-over juices.

On a floured work surface, roll out one of the discs of chilled dough (keep the other one in the refrigerator). Turn the dough about a quarter turn after every few rolls until you have a circle 12 inches in diameter. Carefully place the dough into a 9-inch round pie dish. Tuck it in with your fingers, making sure it is smooth.

Use a slotted spoon to transfer the filling into the crust. Reserve the juice in the bowl for the next step. Refrigerate pie, uncovered, as you reduce the juice in the next step.

Pour the leftover juice into a small saucepan. Cook over low heat, stirring, until juice has slightly reduced and thickened into an applesauce-like consistency, about 1–2 minutes. Cool for 5 minutes, then pour over the filling. Do your best to gently toss together. The reduction will harden and thicken as a result of mixing with the cold fruit—this is normal. Dot the pieces of cold butter on top of the filling. Place the pie in the refrigerator while you roll out the top crust.

Remove the other disc of chilled pie dough from the refrigerator. Roll the dough into a circle that is around 12–15 inches diameter. Using a pastry wheel, sharp knife, or pizza cutter, cut strips of dough; in the pictured pie, I cut 10 1.25-inch-wide strips. Carefully thread the strips over and under one another, pulling back strips as necessary to weave. (Here’s a lattice pie crust tutorial if you need visuals.) To seal the edges, use a small paring knife or kitchen shears to trim excess dough that extends more than 1.5–2 inches over the edge. Fold bottom pie dough edges back over and press/meld into the lattice edges to form a smooth, neat rim. Flute or crimp the pie crust edges with a fork.

Lightly brush the top of the pie crust with the egg wash. Sprinkle the top with coarse sugar, if using.

Bake the pie at 425°F for 20 minutes; then, keeping the pie in the oven, reduce the oven temperature down to 375° (190°C). Place a pie crust shield (see Note for homemade shield) on the edges to prevent them from over-browning. Continue baking the pie until the filling’s juices are bubbling everywhere, including in the center, 45–50 more minutes. This sounds like a long time, but under-baking the pie means a soupy, paste-like filling. If you want to be precise, the internal temperature of the filling taken with an instant-read thermometer should be around 200°F (93°C) when done. Tip: If needed towards the end of bake time, remove the pie crust shield and tent a large piece of foil over the entire pie if the top looks like it’s getting too brown.

Remove pie from the oven, place on a cooling rack, and cool for at least 5 hours before slicing and serving. Filling will be too juicy if the pie is warm when you slice it.

Cover leftovers tightly and store in the refrigerator for up to 5 days.