1. In a large bowl, mix flour and salt; cut in butter until crumbly. Gradually add ice water, tossing with a fork until dough holds together when pressed. Divide dough into 2 portions so that 1 is slightly larger than the other. Shape each into a disk. Cover and refrigerate 1 hour or overnight.
2. For filling, in a large saucepan, whisk sugar, cornstarch, salt, water and, if desired, cinnamon until smooth; add blueberries. Bring to a boil; cook and stir 2 minutes or until thickened. Cool slightly.
3. Preheat oven to 400°. Gently fold raspberries, strawberries, blackberries and lemon juice into blueberry mixture. On a lightly floured surface, roll out larger portion of dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim crust to 1/2 in. beyond rim of plate. Add filling; dot with butter.
4. Roll remaining dough to a 1/8-in.-thick circle; cut into 1/2-in.-wide strips. Arrange over filling in a lattice pattern. Trim and seal strips to edge of bottom crust; flute edge. Bake 10 minutes.
5. Reduce oven setting to 350°; bake 45-50 minutes or until crust is golden brown and filling is bubbly. Cool on a wire rack.
