For the pie dough:

1. Place the flour, butter, and salt in a food processor fitted with the blade attachment (see Note). Pulse about ten times and stop once the butter has blended into the flour with lots of pea-size lumps left. Add the cream cheese and pulse a few more times, just until the cream cheese has blended in but the mixture is still quite lumpy. Sprinkle the water over the surface and pulse a few more times, just until the mixture can be squeezed together into a ball.
2. Shape the dough into a ball by squeezing it together, flatten the ball into a disk, and cover tightly with plastic wrap. Gently tap and roll the sides of the disk on the counter to help them become less jagged. Once they’re smooth and no longer have big cracks, place the disk flat on the counter and gently press and pat the top to make sure it’s nice and flat. As you gently press down on the top, the plastic wrap compresses the sides and ensures that any jagged edges are smoothed out. Refrigerate for 30 minutes. If you chill it completely, let it sit at room temperature for 30 to 50 minutes, just until it’s pliable but still chilled.

For the pistachio frangipane:

1. Combine the pistachios, egg white, sugar, and butter in a stand mixer and beat at high speed with the paddle attachment until light and fluffy, about 2 minutes. Stir in the flour once it’s fluffy. Set aside at cool room temperature.
2. Meanwhile, preheat the oven to 400°F [205°C] and line a rimmed baking sheet with parchment paper.

For the filling:

1. In a medium mixing bowl, fold together the rhubarb, sugar, cornstarch, water, and salt.
2. On a lightly floured surface, roll out the chilled dough to just under ¼ in [6 mm] thick and about 12 in [30.5 cm] across. Rotate occasionally as you work but try not to handle the dough too much and try to keep the edges from cracking too deeply.
3. Once it’s rolled out, gently wrap the dough around your rolling pin to transfer it to the prepared baking sheet. Spread the frangipane in the center of the rolled-out dough, leaving a 2 in [5 cm] border around the edges.
4. Place the rhubarb filling on the frangipane layer and be sure to scrape the bowl with a spatula so you don’t miss anything. Arrange in an even layer, leaving a 2 in [5 cm] border around the edges. Fold the edges over in a rustic overlapping pattern so that the middle stays open and brush the crust’s surface all over with the egg wash.
5. Bake for about 50 minutes, until the whole thing is bubbly and the crust is golden brown.
6. Let cool before slicing (at least 15 minutes, ideally 1 hour). Serve plain or with extra pistachios.