Whisk flour, salt, and sugar in a bowl. Cut in the 1 cup cold butter until pea-sized pieces remain. Stir in ice water a tablespoon at a time until dough just holds together. Divide into two discs, wrap, and chill at least 1 hour.

Roll the larger disc into a 12-inch circle and fit into a 9-inch pie dish; roll the smaller disc into an 11-inch circle. Trim bottom crust overhang to 1 inch.

Toss rhubarb, apples, brown sugar, cornstarch, zest, lemon juice, cinnamon, and salt in a bowl. Let sit 15 minutes, stirring once or twice.

Mound filling into the bottom crust, dot with 2 tbsp butter. Top with the second crust, fold edges under, crimp to seal, and cut steam vents.

Preheat oven to 425°F with rack in lower third. Place pie on a foil-lined sheet and bake 15 minutes. Lower to 375°F and bake 40 to 50 minutes more, until filling bubbles thickly and crust is deep golden. Cover edges with foil if browning too fast.

Cool on a rack at least 2 to 3 hours, then slice and serve.