First, combine the butter, flour, and salt. Use a pastry blender to cut in the butter until the mixture becomes very crumbly. Gradually add the chilled water, a few tablespoons at a time, mixing with a fork until the dough starts to come together. Shape it into a disk, wrap in plastic, and chill for at least 2 hours.

In another bowl, whisk together the flour, granulated sugar, and brown sugar. Cut in the butter with a pastry blender until the mixture is crumbly. Set this streusel topping aside.

For the filling, mix together the rhubarb, strawberries, granulated sugar, brown sugar, cornstarch, vanilla extract, and lemon juice until well combined.

On a lightly floured surface, gently roll out the pie crust to a 12-inch circle. Transfer the crust to a pie plate by folding it in half or rolling it around your rolling pin. Carefully fit the crust into the pie pan, trimming the edges about an inch over the side, and then tuck in the excess dough and flute or crimp the edges.

Spoon the fruit filling into the prepared pie crust. Sprinkle the streusel crumb topping evenly over the top. Place the pie on a rimmed baking sheet to catch any spills.

Bake in a preheated 375°F oven for 50-55 minutes, or until the pie is golden brown and the filling is bubbling. If the crust starts to over-brown, cover it loosely with aluminum foil.

Let the pie cool completely before slicing and serving.