- Preheat the oven to 375°F (190°C). Pull the pie dough from the fridge 30 minutes before rolling. Dust a 9-inch (23 cm) pie plate with flour and place it on a baking tray lined with parchment paper.
- Toss the rhubarb and strawberries together in a large bowl. In a separate bowl, stir the sugar and tapioca starch together, then add this mixture to the fruit and stir well.
- On a lightly floured surface, roll out the first disc of dough to a circle just less than ¼ inch (6 mm) thick (1). Line the pie plate with the pastry, leaving the edges untrimmed. Sprinkle the oats over the pastry and then spoon the fruit into the crust (2).
- Roll out the second disc of pastry to about ⅛ inch (3 mm) thick and in more of a square. Use a knife or pastry wheel to cut long strips about ½ inch (1.2 cm) wide (you should have between 12 and 16 strips) (3).
- For the lattice, place half of the strips parallel to each other over the fruit, leaving ½ inch (1.2 cm) between them. Starting at one edge, gently lift alternating strips of pastry, folding them halfway back. Set a new strip of pastry beside the folds and perpendicular to them. Unfold the original strips over the newly laid one (4). Lift the opposite alternating strips and fold them back as far as they can go (to the edge of the newly laid strip). Set a second perpendicular strip beside the first one and then unfold the original ones (5). Repeat this technique, moving first toward one side of the pie shell and then the other, until the lattice is complete (6).
- Trim any excess pastry from the top and pinch the edges to create a fluted design. Brush the top of the lattice with the egg and bake the pie for about 75 minutes, until the fruit is bubbling and the pie is a rich golden brown.
- Cool the pie on a cooling rack for at least 2 hours before serving, or chill to serve cold. The pie will keep, loosely wrapped, in the fridge for up to 2 days.
Sprinkling rolled oats over the bottom pastry crust creates an absorbing barrier between the fruit and the pastry. As the pie bakes, the oats soak up some of the juices and thicken them, helping to keep the crust crisp.
Fruit pies made with juicy fruits like rhubarb and berries need some added thickening to hold the filling together. Flour has a dull finish and never really loses its raw taste. Cornstarch needs to reach a full 212°F (100°C) throughout to fully activate. Tapioca starch, however, has stronger thickening power, activates at a lower temperature (140°F/60°C) and thickens evenly.
Recipe from Baking Day with Anna Olson, Appetite by Random House, 2020