01 -
Whisk flour, sugar, and salt in a large bowl. Cut in the cold butter until the pieces are mostly pea-size, then stir in ice water 1 tablespoon at a time until the dough holds when pressed. Divide into 2 disks, wrap, and chill for at least 1 hour.
02 -
Combine rhubarb, strawberries, sugar, cornstarch, lemon juice, lemon zest, vanilla, salt, and butter in a saucepan. Cook over medium heat for 6-8 minutes, stirring often, until the juices look glossy and lightly thickened. Cool to room temperature.
03 -
On a floured surface, roll one dough disk into a 12-inch round. Fit it into a 9-inch pie plate, leaving a small overhang. Refrigerate while you roll the top crust.
04 -
Roll the second dough disk into a 12-inch round and cut it into strips. Spoon the cooled filling into the bottom crust, weave the strips over the filling, trim the edges, and crimp firmly.
05 -
Refrigerate the assembled pie for 20 minutes. Heat the oven to 425°F. Brush the lattice with egg wash and sprinkle with coarse sugar if using.
06 -
Bake at 425°F for 20 minutes. Reduce the oven to 375°F and bake for 35-40 minutes more, until the crust is deep golden and the filling bubbles thickly in the center. Shield the edges with foil if they brown too fast.
07 -
Cool the pie on a rack for at least 4 hours before slicing. Serve at room temperature or slightly warm.