Prepare pie dough by combining flour, sugar, and salt. Cut in cold butter until mixture resembles coarse crumbs. Gradually add ice water until dough comes together. Divide in half, shape into discs, wrap, and refrigerate for 1 hour.

Preheat oven to 400°F (200°C). Position rack in lower third of oven.

In a large bowl, combine rhubarb and apples. In another bowl, whisk together sugar, flour, cornstarch, cinnamon, and nutmeg.

Pour sugar mixture over fruit and toss to coat evenly. Let sit for 10 minutes.

Roll out one dough disc to ⅛-inch thickness and transfer to 9-inch pie plate. Pour filling into crust and dot with butter pieces.

Roll out second dough disc and cut into strips for lattice top. Weave strips over filling and crimp edges to seal.

Brush lattice with egg wash and sprinkle with coarse sugar.

Place pie on baking sheet. Bake at 400°F for 20 minutes, then reduce to 350°F and bake 40-50 minutes until golden and bubbling.

Cool on wire rack for at least 4 hours before slicing to allow filling to set properly.