In a large mixing bowl, combine the flour, sugar, and salt. Cut in cold butter until the mixture resembles coarse crumbs. Gradually add cold water, mixing until a dough forms. Split the dough in half, shape into discs, and chill in the refrigerator for at least 1 hour.

In a separate large bowl, mix together the strawberries, rhubarb, sugar, cornstarch, lemon juice, and vanilla extract. Toss everything together gently. Let the mixture sit while you roll out the pie crust.

Preheat your oven to 400°F (200°C). Roll out one chilled pie crust disc into a 12-inch circle and transfer it into a 9-inch pie pan. Pour the strawberry rhubarb filling into the bottom crust and dot with butter.

Roll the second chilled crust into a 12-inch circle and place it over the filling. Cut slits in the top crust for ventilation. Crimp the edges to seal, brush with egg wash, and sprinkle with turbinado sugar.

Bake the pie in the preheated oven for 20 minutes, then reduce the temperature to 350°F (175°C) and continue baking for another 40-50 minutes, or until the filling is bubbly.

Allow the pie to cool completely for 3-4 hours before slicing. Serve warm, possibly with vanilla ice cream or whipped cream.