Instructions

1. Arrange a rack in the middle of the oven and heat to 375°F. Place a rimmed baking sheet in the oven to preheat. Unroll one of the pie crusts and transfer to a 9-inch pie plate. Press the dough into the bottom and up the sides of the pie plate, being careful not to puncture the dough.
2. Using a fork, whisk 1 large egg and 1 teaspoon water together in a small bowl to make an egg wash. Brush the the pie dough with an even layer of the egg wash. Refrigerate the pie dish and remaining crust while preparing the filling.
3. Place 1 1/2 pounds chopped rhubarb, 3/4 cup granulated sugar, 1/4 cup cornstarch, 1/2 teaspoon ground cinnamon (optional), and 1/4 teaspoon salt in a large bowl and toss until well combined. Pour all of it, including any liquid, into the prepared crust.
4. Unroll the second pie dough on a work surface. Use a 1-inch round cutter or paring knife to make 8 to 10 vent holes in the dough. (Alternatively, use a straw to create tiny vent holes.) It is important that the filling has plenty of holes to vent, or the filling won’t thicken.
5. Transfer the second crust onto the pie over the filling and crimp the edges to seal. Brush the top and edges of the dough with the egg wash. Sprinkle generously with the 2 tablespoons coarse sugar (you might use not all of it).
6. Bake until the filling is bubbling, 40 to 45 minutes. Check after 30 minutes -- you may need to cover the top or edges of the pie crust with aluminum foil if the crust is getting too dark. Let the pie cool on a wire rack for at least 4 hours before serving.