Chop the rhubarb into small pieces and combine it with brown sugar, cinnamon, and nutmeg in a medium saucepan. Cook over medium heat for about 15-20 minutes until the rhubarb softens.

In a large bowl, whisk together flour and salt. Create a well in the center for the butter and cut it in using a pastry cutter until it resembles coarse crumbs. Gradually add ice water and stir until dough forms. Refrigerate for at least 30 minutes.

Preheat your oven to 375°F (190°C). Roll out half the dough on a lightly floured surface to fit your pie dish. Place the rolled dough into the dish and add the filling.

Roll out the remaining dough to cover the filling or create a lattice pattern. Brush the top with a beaten egg.

Bake for about 40 minutes until the crust is golden brown and the filling bubbles up. Cool on a wire rack for at least 30 minutes before serving.