To Make the Crust:

1. Combine flour, salt, and butter in a large mixing bowl.
2. Cut together with a pastry cutter until mixture takes on a crumbly appearance.
3. Stir in water 1 tablespoon at a time with a fork using a tossing motion, until the dough forms into a ball. You might not need to use all of the water. Divide in half, and form into two flattened discs. Set aside, or wrap in plastic and refrigerate until ready to use.

To Make the Pie:

1. Preheat oven to 400°F and move rack to the middle of the oven.
2. In a large bowl, combine rhubarb, strawberries, sugar, cornstarch, lemon juice, and cinnamon. Set aside.
3. On a lightly floured surface, roll out one of the discs of dough to fit a 9-inch deep dish pie pan.
4. Line the pan with the bottom crust, then fill with the strawberry rhubarb mixture.
5. Roll out the remaining dough and cut into 1-inch-wide strips. Place on top of the filling, alternating to form a lattice top. Crimp edges with your fingers.
6. Beat an egg in a small bowl, and brush onto the latticework crust with a pastry brush. Sprinkle crust with sugar if desired.
7. Bake for 15 minutes.
8. Reduce heat to 350°F, and continue to bake for another 45 minutes. The pie is done when the filling has thickened and the crust is golden brown.
9. Remove from the oven and place on a cooling rack to cool and set for at least 20 minutes before serving.