Wash rhubarb and strawberries. Trim rhubarb and cut into ½-inch pieces. Hull and quarter strawberries. Combine fruits with half of the sugar in a large bowl and let sit for 15-30 minutes.

Strain the fruit mixture to eliminate excess juice. Transfer back to bowl.

Gently combine drained fruit with tapioca starch, remaining sugar, salt, lemon juice, and vanilla extract, if using.

Dust surface with flour and roll out bottom crust into a 12-inch circle. Fit into greased 9-inch pie dish.

Spoon filling into the crust and dot with small pieces of cold butter. Crimp edges to seal.

Brush top crust with egg wash and optionally sprinkle coarse sugar on top.

Preheat oven to 425°F (220°C) and bake for 15 minutes. Reduce temperature to 350°F (175°C) and bake for an additional 45-55 minutes.

Cool pie on wire rack for at least 3 hours before slicing.