In a large mixing bowl, combine the flour and salt. Cut in the shortening using a pastry cutter or fork until the mixture resembles coarse crumbs. Add ice water, one tablespoon at a time, tossing with a fork until the dough just comes together.

Divide the dough into two portions, one slightly larger than the other. Shape each into a disk, wrap in plastic, and chill for at least 30 minutes.

In a separate bowl, gently toss the strawberries, blueberries, and raspberries with cornstarch and apple juice. Let the mixture rest for 15 minutes. Stir in the sugar and cinnamon just before filling the pie.

Preheat your oven to 450°F (230°C). Roll the larger pastry disk into a 12-inch circle on a lightly floured surface. Fit it into a 9-inch pie pan, leaving about a 1-inch overhang.

Spoon the berry filling into the crust and dot with butter pieces.

Roll out the smaller dough disk into an 11-inch circle and cut into ½-inch strips. Twist each strip slightly and arrange in a lattice pattern over the filling. Trim excess dough and crimp the edges to seal.

Place the pie on a baking sheet. Bake at 450°F for 15 minutes, then reduce heat to 350°F (175°C) and bake for another 55 minutes, or until the filling is bubbly and the crust is golden brown.

Let the pie cool slightly before serving. Enjoy warm with a scoop of vanilla ice cream or a dollop of whipped cream.