- In a mixer fitted with a paddle attachment, combine flour, sugar and salt. Cut butter into small pieces (you can pull the butter from the fridge 30 minutes before adding) and add to flour, mixing until dough is a rough, crumbly texture. Whisk egg, water and vinegar and add at once, mixing until dough comes together. Shape dough into 2 discs, wrap in plastic and chill for at least one hour.
- While dough is chilling, prepare the fruit. Toss the rhubarb and strawberries with the tapioca and let sit for 5 minutes, so that tapioca can soften. Stir in the sugar and vanilla.
- Pull out the pastry from the fridge 20 to 30 minutes before rolling. Preheat the oven to 400°F (200°C). Place a 9-inch (23 cm) pie plate onto a parchment lined baking tray and dust the pie plate with flour. On a lightly floured work surface, roll out the first pastry disc to just under ¼-inch (.75 cm) thick and line the prepared pie plate. Sprinkle the oats on the bottom of the pastry shell. Spoon the filling into the pie shell. Roll out the remaining pastry disc and cut out a small hole in the centre of the pastry (so steam can escape as the pie bakes). Place the rolled pastry overtop the fruit and press the edges together. Trim away any excess dough and pinch the edges to create a pattern (or use a fork to press the pastry edges together). Brush the surface of the pastry with the egg mixture and sprinkle the pie with turbinado sugar. Bake the pie for 12 minutes at 400°F (200°C), then reduce the heat to 375°F (180°C) and bake for about 40 minutes more, until the crust is a rich golden brown and the filling is bubbling. Cool the pie for at least 2 hours before slicing.
The pie can be made a day ahead and served chilled.