Prepare the pie crust: Mix flour, sugar, and salt in a large bowl. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Add ice water one tablespoon at a time, mixing until the dough just comes together. Divide into two disks, wrap in plastic, and refrigerate for at least 30 minutes.

Make the filling: Combine rhubarb and apples in a large bowl. In a separate bowl, whisk together sugar, flour, cinnamon, nutmeg, and salt. Pour the dry mixture over the fruit and add lemon juice. Toss everything together gently until the fruit is evenly coated.

Roll out the bottom crust: On a floured surface, roll one disk of dough into a 12-inch circle. Transfer it to a 9-inch pie plate, letting the excess hang over the edges. Don't trim it yet!

Fill the pie: Pour the fruit mixture into the prepared crust, mounding it slightly in the center. Dot the top with small pieces of cold butter.

Add the top crust: Roll out the second disk of dough and place it over the filling. You can create a lattice pattern or simply cover it completely with vents cut in the top. Trim the excess dough, leaving about ½ inch overhang, then crimp the edges together.

Apply egg wash: Brush the top crust with beaten egg and sprinkle with coarse sugar if desired. This creates a beautiful golden finish.

Bake the pie: Preheat your oven to 425°F. Place the pie on a baking sheet (to catch any drips) and bake for 20 minutes. Reduce the temperature to 350°F and continue baking for 40-50 minutes until the crust is golden brown and the filling is bubbling.

Cool completely: Let the pie cool on a wire rack for at least 3-4 hours before slicing. I know it's tempting to dig in right away, but the filling needs time to set properly!