Prep tip: I like to prepare the two pie crusts and berry filling a day ahead. Refrigerate both until ready to assemble. The colder everything is before baking, the better!
In a large pot, combine berries straight from freezer, sugar, salt, and lemon juice (if using). Simmer over medium heat until warm and juicy, about 5–10 minutes, stirring gently occasionally.
Remove 1/3 cup of juice from the pot to a bowl. Add in cornstarch and whisk until fully smooth. Gradually stir this mixture into the simmering berries. Continue stirring gently until it thickens to a jam-like consistency, about 5 minutes.
Remove from heat. Stir in butter until melted, then add vanilla. Taste and adjust sweetness, adding more sugar if needed. Allow to cool to room temperature.
Place one pie crust in a 9-inch deep dish pie pan. Pour cooled berry filling into the crust, smoothing in an even layer. Cover with the second crust. Cut steam vents or create a lattice top (see note 2). Crimp edges to seal (see note 3).
Combine egg and milk. Brush over the crust (you won’t use it all) and sprinkle with coarse (or regular) sugar.
Place the pie on a sheet pan. Bake at 400°F for 40–50 minutes. If crust browns too quickly, cover with a pie shield. Cool on a wire rack for 3–5 hours.
Enjoy pie with fresh whipped cream or vanilla ice cream.
Notes
Note 1: Wyman’s® Triple Berry Blend, including wild blueberries, blackberries, and raspberries is my favorite for this pie. If using fresh berries, equally mix blueberries, blackberries, and raspberries.Note 2: Lattice crust instructions
Roll out: On a lightly floured surface, roll the crust into a circle 1–2 inches larger than the pie pan.
Cut strips: Use a very sharp knife to cut strips of desired width.
First layer: Place the longest strip in the center. Add shorter strips on either side, spaced evenly.
Weave: Alternately fold back even and odd-numbered strips. Lay a strip perpendicular across the pie. Unfold the folded strips over this new strip.
Repeat weaving: Continue weaving until the pie is fully covered.
Trim and seal: Trim excess dough. Pinch the lattice top and bottom crusts together, crimping the edges.
Note 3: To crimp the edges of a pie crust, pinch the dough (from the top and bottom crusts) between your thumb and forefinger around the rim of the pie pan, forming a consistent, wavy pattern.Nutrition Note: Nutrition information includes the two crusts, but is approximate, since crust ingredients can vary.Storage: Let pie cool completely. Cover loosely, and store for up to two days. refrigerate for up to four days, covered tightly. To freeze, wrap well and freeze for up to four months. Thaw in fridge.