In a large mixing bowl, combine all-purpose flour, salt, and sugar. Cut in shortening until mixture resembles coarse crumbs. Gradually add cold water until a dough forms. Divide into two discs, wrap in plastic wrap, and refrigerate for at least 30 minutes.

In another bowl, toss mixed berries with granulated sugar, Clearjel starch, salt, orange juice, and lemon juice until evenly coated. Allow to sit to meld flavors.

Preheat oven to 375°F (190°C). Roll out one disc of dough and fit into a pie dish. Pour berry filling into the crust, mound it in the center. Roll out second disc and cut into strips for a lattice pattern on top. Seal edges.

Whisk egg and water for egg wash. Brush over the lattice top and crust edges. Sprinkle sugar over lattice. Cut slits in the crust for steam to escape.

Place pie on a baking sheet and bake for 45-50 minutes or until the crust is golden and filling is bubbling. Cover edges with foil if browning too quickly.

Cool pie on a wire rack for at least 20 minutes before serving. Serve warm, preferably with a scoop of vanilla ice cream.