A flaky, buttery double-crust pie filled with juicy strawberries and tart rhubarb, baked into a thick, jammy filling that slices cleanly—no runny mess.
Prepare the pie crust dough (enough for a bottom and a top) through the chilling step; keep the dough well chilled until needed.
Make the filling: In a large bowl, combine the rhubarb, strawberries, brown sugar, granulated sugar, cornstarch, salt, orange juice, and vanilla; toss to coat. Set aside while you roll the dough.
Optional thickening step: If juices pool in the bowl, pour the juices into a small saucepan and cook over medium-low heat until thick and syrupy; let cool about 5 minutes, then pour over the fruit and gently stir to distribute.
Roll out the bottom crust and fit it into a pie dish. Spoon in the fruit filling and dot the top with the cold butter cubes.
Roll out the top crust and create a lattice (or use a full top crust). Crimp or flute the edges to seal.
Brush the crust with egg wash and, if desired, sprinkle with coarse sparkling sugar.
Place the pie on the middle oven rack with a parchment-lined baking sheet on the rack below to catch drips. Bake at 400°F (204°C) for 20 minutes.
Reduce the oven temperature to 375°F (190°C); add a pie crust shield if needed, and continue baking until the filling bubbles through the lattice and around the edges and the internal temperature reaches 200–212°F.
Remove from the oven and cool the pie completely for several hours; this cooling time is essential for the filling to fully set.
Slice and serve once fully cooled.