Make the crust dough.
Cut the shortening into the flour and salt until the mixture looks like fine crumbs. Add the ice-cold water and gently bring the dough together, handling it as little as possible. Divide it into two equal portions for the top and bottom crust.
Prepare the fruit filling.
Combine the chopped rhubarb and strawberries in a large bowl. In a separate bowl, mix the sugar and flour together.
Roll out the bottom crust.
Fit one piece of dough into a deep 9-inch pie plate.
Add the moisture-protecting layer.
Sprinkle 1/4 cup of the sugar-and-flour mixture into the bottom crust before adding the fruit. This helps reduce sogginess in the base.
Fill the pie.
Add the fruit to the crust, then sprinkle the remaining sugar-and-flour mixture over the top so it settles into the fruit.
Top and finish the crust.
Roll out the second piece of dough and lay it over the filling. Crimp the edges, cut steam vents in the top, brush with milk, and sprinkle with sugar. Set the pie on a foil-lined baking sheet to catch any bubbling juices.
Bake in two stages.
Bake at 425°F for 15 minutes, then reduce the heat to 325°F and continue baking for 1 hour. Check the pie around the 30-minute mark, and if the crust is browning too quickly, tent it loosely with foil.
Cool before slicing.
Let the pie cool for several hours so the filling can set. Serve once the center is no longer runny