Step 1

Preheat the oven to 375°. Place a foil-lined rimmed baking sheet on the center rack of the oven to preheat.

Step 2

Roll out 1 piece of dough on a floured surface into a 12-inch round, starting at the center and working your way out. (Sprinkle flour over the top of the dough if it's a bit too moist.) Carefully lift the dough and place in a 9-inch pie pan (not deep dish); gently press against the sides of the pan. Trim excess overhanging dough at the edge of the pie plate.

Step 3

Stir together the sugar, cornstarch, and salt in a large bowl until evenly combined. Add the rhubarb, strawberries, orange juice, orange zest, and vanilla. Toss until evenly coated. Pour the fruit mixture into the pie crust.

Step 4

Roll out the other piece of dough into a 12-inch round on a floured surface. Cut into eight 1 1/2-inch-wide strips. Arrange 4 strips of dough in parallel rows on top of the filling. Working with one strip at a time, arrange the remaining 4 strips perpendicular to the first ones, weaving the strips over and under to form a lattice. Trim the ends of the strips and press them against the bottom crust. Press with a fork all around the edge of the pie crust to crimp.

Step 5

In a small bowl, whisk the egg and 1 tablespoon of water with a fork. Brush the dough lightly with the egg wash and sprinkle with the turbinado sugar.

Step 6

Bake the pie until the filling is bubbly and thickened in the middle of the pie, 1 hour 20 minutes to 1 hour 30 minutes. (Cover the pie with foil if the crust is getting too dark after 45 minutes.) Allow the pie to cool to room temperature before slicing, about 4 hours.