In a large bowl, add rhubarb, strawberries, and 1 cup of sugar and toss to combine. Let sit at room temperature until the fruit releases its juices, about 1 hour.
Meanwhile, line a 9-inch pie plate with 1 pie crust, letting the excess dough hang over the edge. Cover with plastic wrap and refrigerate while the fruit rests.
Using a pizza wheel or pastry wheel, cut the second pie crust into even strips for a lattice top, about 1/2-inch to 3/4-inch wide strips. Refrigerate while assembling the pie.
Adjust an oven rack to the lowest position and preheat oven to 425 degrees. Line a rimmed baking sheet with foil.
Drain fruit well reserving 1/4 cup of juice. In a large bowl, add fruit, reserved fruit, cornstarch, vanilla, and salt and toss to combine. Taste fruit and add more sugar to taste if desired.
Pour fruit into prepared pie crust, mounding it in the middle. Lay 4 to 7 parallel strips of pie crust about 1/2-inch apart to form the first layer of lattice. Fold back every other strip.
Place one long strip of pie crust perpendicular across all parallel strips. Unfold the folded parallel strips over the perpendicular strip.
Fold back parallel strips running under perpendicular strip. Lay another perpendicular strip next to the first, about 1/2-inch apart. Unfold the folded parallel strips over the perpendicular strip.
Continue adding perpendicular strips, folding back the parallel strips then unfolding, until the entire top of the pie crust is covered with lattice. Crimp the strips to the lower crust to seal the edges.
In a small bowl, beat egg and milk. Brush on crust and sprinkle with remaining 1 tablespoon sugar.
Set pie on baking sheet place in oven. Bake until the top crust is golden, about 25 minutes.
Reduce the oven temperature to 375 degrees, rotate the baking sheet, and continue bake until the juices are bubbling and the crust is deep golden brown, 25 to 35 minutes longer.
Let the pie cool on a wire rack until the filling has set, about 2 hours; serve slightly warm or at room temperature.