Prepare pie dough as instructed in the Flaky Spelt Pie Pastry recipe up to and including step 3.
Preheat the oven to 425 degrees F and line a large baking sheet with parchment paper. Set aside.
Mix the rhubarb, sugar, spelt flour, nutmeg and orange zest in a large bowl. Set aside.
Roll out one piece of pie dough into a 16" circle (or 2" wider than the width of the baking sheet). Trim the edges with a knife or pastry cutter if desired. Gently fold the dough in half and then in quarters, transfer it to the baking sheet and unfold it. Pile the rhubarb mixture in the center of the dough leaving an approximately 2" border. Fold the edges over the mixture. Sprinkle the pecans on top of the visible rhubarb mixture. Brush the edges of the dough with milk and sprinkle generously with sugar.
Bake for 40 to 50 minutes or until the pastry is golden brown and the rhubarb is soft. Remove it from the oven and allow the galette to cool on the baking sheet until the mixture stops bubbling, then transfer it to a wire rack to cool completely before serving.