Prepare either pie crust recipe through step 5, including chilling for at least 2 hours. I usually make the crust the night before. The pie crust recipe makes 2 crusts and you’ll be using both crusts.

Place 1/2 cup of each type of berry in a medium bowl and set aside.

Place the remaining 2 and 1/2 cups blueberries, 1 cup raspberries, and 1 cup blackberries in a medium saucepan set over medium heat. Add the sugar and cinnamon and cook, stirring frequently, until the berries begin to release some juice. Place the cornstarch and lemon juice in a small bowl. Remove 3 to 4 Tablespoons of warm berry juice from the saucepan, and add it to the bowl of cornstarch. With a fork, whisk together the cornstarch and juices until cornstarch is dissolved and you have a smooth paste. Stir the cornstarch mixture into the saucepan with the berries. Cook, stirring occasionally, until the mixture is simmering and starting to thicken into a jam consistency, about 5–7 minutes; if you have an instant-read thermometer, the filling is done cooking when it reaches about 185°F (85°C). Remove from heat and stir in the reserved berries (1/2 cup of each), lemon zest, and vanilla. Set aside to cool at room temperature for 20 minutes or up to 1 day. If not using after 20 minutes, cover lightly.

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 berry juices.

On a floured work surface, roll out one of the discs of chilled dough. (Keep the other disc in the refrigerator until needed.) 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 pie dish. Tuck it in with your fingers, making sure it is smooth.

Spoon the filling (it’s ok if it’s still warm) into the crust and place the pie in the refrigerator as you roll out the top crust.

Remove the other disc of chilled pie dough from the refrigerator. Roll the dough into a circle 12 inches diameter. Using a pastry wheel, sharp knife, or pizza cutter, cut strips of dough; in the pictured pie, I cut 10 1-inch-wide strips. Remove the pie from the refrigerator and carefully thread the dough strips over and under one another, pulling back strips as necessary to weave. (Here’s a lattice pie crust tutorial if you need visuals.) Press the edges of the strips into the bottom pie crust edges to seal. Use a small paring knife or kitchen shears to trim off excess dough. Flute or crimp the pie crust edges with a fork.

Lightly brush the top and edges of the pie crust with 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°F (190°C). Place a pie crust shield (see Notes 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, 45–55 more minutes. If you want to be precise, the internal temperature of the filling taken with an instant-read thermometer should be about 200°F (93°C) when done. Tip: If needed towards the end of bake time, if the top looks like it’s getting too brown, remove the pie crust shield and tent an entire piece of foil over the pie.

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.