Preheat your oven to 450°F (230°C) and slide the rack to the lowest position.

Roll out one pie crust and press it into a 9-inch pie dish, letting the edges hang slightly over.

In a large bowl, combine chopped rhubarb, sugar, cornstarch, salt, and lemon zest. Toss until every piece is coated in the sweet-tart mix.

Pour the rhubarb filling into the prepared crust, dot with butter pieces, then top with the second crust. Crimp the edges to seal and slice vents in the top for steam to escape.

Bake at 450°F for 10 minutes to set the crust, then lower the heat to 350°F and bake 40–45 minutes more, until the filling bubbles and the double crust turns golden brown.

Remove the pie from the oven and let it cool on a wire rack for at least 2 hours before slicing.