Instructions: Prepare the Crust: In a large bowl, combine the flour, salt, and sugar.Add the cold, cubed butter and use a pastry cutter or fork to cut the butter into the flour mixture until it resembles coarse crumbs.
Prepare the Filling: In a large bowl, combine the sliced rhubarb, sliced strawberries, granulated sugar, cornstarch, lemon juice, vanilla extract, and ground cinnamon (if using). Toss gently to combine.
Assemble the Pie: Roll out the second dough disk into another 12-inch circle. Place it over the filling and trim any excess dough, leaving about a 1-inch overhang. Fold the overhang under itself and crimp the edges to seal.
Bake: Place the pie on a baking sheet to catch any drips. Bake for 20 minutes at 400°F (200°C), then reduce the oven temperature to 350°F (175°C) and bake for an additional 40-50 minutes, or until the crust is golden and the filling is bubbly.
Cool: Allow the pie to cool completely on a wire rack before serving. This helps the filling set.
Serving: Serve the blackberry pie warm or at room temperature, optionally with a scoop of vanilla ice cream or a dollop of whipped cream. Enjoy!