Preheat oven to 425 degrees F. Roll out half the pastry and line a 9 inch pie plate. Trim leaving ½ inch over the pie plate.
Combine rhubarb strawberries, sugar, cornstarch, lemon juice and cinnamon. Place in the pie shell.
Roll out remaining pastry and cut into 1 inch strips. Make a lattice top crust on the pie by crisscrossing the strips over the filling. Trim the strips even with the pie plate. Fold the ½ inch of the bottom crust over the ends of the strips. Seal and flute the edges. Brush lattice with egg.
Place on a baking sheet or tinfoil in the oven and bake for 15 minutes, until the crust starts to brown. Reduce heat to 375 degrees and continue to bake for 50 to 60 minutes, until crust is golden, rhubarb is tender, and filling is thickened.