STEP 1: To make the pastry pulse the flour and salt in a food processor until well blended. Add the shortening and pulse until the mixture resembles coarse crumbs. Sprinkle 1 tablespoon cold water over the mixture and pulse 2 or 3 times. Repeat the process with the remaining cold water as needed, 1 tablespoon at a time, following each addition of water by 2 quick pulses, just until the dough comes together when a small bit is pressed between your fingers. Divide the dough into 2 pieces, one slightly larger than the other, and shape into disks. Refrigerate for at least 1 hour, or overnight, wrapped in wax paper.
STEP 2: Preheat the oven to 425°F. Have ready a 9-inch pie plate.
STEP 3: To make the filling combine the sugar, flour, cinnamon, and salt in a large bowl. Stir in the rhubarb, strawberries, and lemon juice.
STEP 4: On a lightly floured surface, roll out the larger pastry disk to a 1/8-inch-thick round. Transfer the dough to the 9-inch pie pan. Trim the edges leaving a 1-inch overhang. Spoon the filling into the shell, top with any sugar mixture remaining in the bowl, and dot with the butter.
STEP 5: Roll out the remaining disk of dough to a 1/8-inch-thick round. Cut into ¾-inch strips and arrange in a lattice pattern on top of the filling. Fold the edge of the bottom crust up over the end of the strips, building a high rim. Seal and crimp decoratively. Place the pie on a baking sheet.
STEP 6: Bake for 15 mintues. Reduce the oven temperature to 350°F and bake for 45 to 50 minutes longer, until the berries are bubbling and the pastry is golden brown. Cool completely on a wire rack.
Step 7: Serve, cut into wedges, with the ice cream, if using.