In a bowl, mix the rhubarb with ⅛ cup of the sugar and then transfer them in a colander to remove some water before using.

In a mixing bowl mix together the flour, sugar, and salt. Cut in butter using the mixer by pulsing until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Add water a tablespoon at a time, pulsing until dough is crumbly but holds together when pressed. Press the dough together to form a ball.

Place the pie dough on a floured surface or on a rolling mat and roll it out thinly to a 14-inch round. Gently fold the round dough in half and then in a quarter. Place the dough on a 9-inch pie form with the pointed part exactly at the middle. Unfold the pie dough and gently fit dough into bottom and sides of the plate without tearing it.. Using kitchen shears, trim dough to a 1-inch overhang; fold under, and press gently to seal. Crimp edges if desired. Refrigerate 30 minutes to an hour.

While waiting, prepare the filling by tossing together the rhubarb, blueberries, the remaining sugar and cornstarch in a bowl. Set aside until ready to use.

In a medium bowl, mix flour, brown sugar, and cinnamon. With your hands, work in butter until large, moist clumps form. Chill, covered, until ready to use.

Preheat oven to 400°F/180°C. Pour the filling into the pie shell and sprinkle evenly with Crumble Topping. Bake with the rack at lowest level until topping and crust is lightly browned, about 40-50 mins. (If topping or crust begins to brown too quickly, tent with foil.) Cool completely before serving.