**Prepare the Pie Crust (if making from scratch):** In a large bowl, whisk together the flour and salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Gradually add ice water, 1 tablespoon at a time, mixing until the dough just comes together. Form into a disc, wrap in plastic, and chill for at least 30 minutes.

Preheat oven to 375°F (190°C). On a lightly floured surface, roll out the chilled pie dough into a 12-inch circle. Carefully transfer the dough to a 9-inch pie plate. Trim and crimp the edges as desired. Set aside.

**Prepare the Filling:** In a large bowl, gently combine the chopped rhubarb, sliced strawberries, granulated sugar, cornstarch, vanilla extract, and optional ground cinnamon. Mix until the fruit is evenly coated.

**Prepare the Crumb Topping:** In a medium bowl, combine the flour, granulated sugar, brown sugar, and optional ground cinnamon. Cut in the cold butter using a pastry blender or your fingertips until the mixture forms coarse crumbs.

Pour the fruit filling into the prepared pie crust. Sprinkle the crumb topping evenly over the fruit filling.

Bake for 50-60 minutes, or until the filling is bubbling and the crumb topping is golden brown. If the crust edges are browning too quickly, cover them loosely with aluminum foil. Let the pie cool completely on a wire rack for at least 3-4 hours before slicing and serving. This allows the filling to set properly.