Preheat the oven to 425 degrees F.

Place the pie dough onto a lightly floured surface and roll out into a 12 inch circle.

Once your dough is rolled out, fold the dough in half, carefully not creasing the center. Pick it up and place into a 9 inch pie plate. Unfold the dough and gently push the dough into the bottom and sides, working out all of the folds. Leave the overhang for the moment.

In a large bowl, add in the sugar and flour. Stir until combined.

Add in the sliced rhubarb and stir until the sugar and flour mixture coats all of the rhubarb.

Pour the filling into the pie crust and push around to fill all the way to the edges of the crust.

Roll out the top pie crust on a lightly floured surface to be about 10 1/2 to 11 inches.

In a small bowl, add in the egg and water. Beat with a fork or whisk until combined. Using a pastry brush, brush the egg wash onto the bottom edge of the pie crust.

Fold the top layer of pie crust in half and lay it on top, paying attention where the center of the crust is and lining it up to the center of the filling. Unfold and gently press the top and bottom dough together around the edges. Cut off the excess dough using a paring knife, leaving about ½ inch overhang.

Tuck the ½ inch of pie crust that hangs over the side of the pie plate underneath the crust so the crust edge is even with the edge of the pie plate.

Crimp the edges with your fingers or a fork.

Using a paring knife, cut 5 to 7 slits in the top of the pie crust for steam holes.

Using the pastry brush, brush the top crust with the egg wash.

Place the pie on a large sheet pan lined with tin foil to catch any filling that spills over.

Place in the preheated oven for 20 minutes at 425 degrees F and then reduce the temperature to 350 degrees F and bake for another 50 minutes until the pie filling is bubbling and the crust is golden brown.