Preheat stovetop to medium heat. Chop rhubarb and apples and mix in a saucepan with brown sugar, cinnamon, and nutmeg. Cook until thickened, about 10-15 minutes.

Combine all-purpose flour and salt in a mixing bowl. Cut in cold butter until crumbly. Add beaten egg and ice water gradually until dough forms. Chill for 30 minutes.

Roll out chilled dough and fit it into a pie dish. Pour in the filling evenly.

Roll out the second dough section, cut into strips, and weave over the filling. Seal edges and brush with beaten egg and sugar.

Preheat oven to 425°F. Bake for 40-50 minutes until golden and bubbling.

Remove from oven and cool on a wire rack for about 2 hours before serving.