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. Crimp edges to seal.
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.