Make the dough in stand mixer using paddle. Put all the flour and half the butter in and turn on setting 3. Mix until consistency of heavy gravel. Then add remaining butter, water and salt and mix until dough ball formed.
Divide into 2 balls then press to make roughly 6 inch by 1 inch dough disks. Wrap in film and refrigerate for at least an hour.
Peel and slice potatoes thin prefferably with mandolin.
Pre Heat oven to 425 degrees
Toss to completely coat in sour cream, pepper, nutmeg, garlic and green onions.
Once dough has set for at least an hour heavily flour surface and roll out to 12 inches approx. Grease 9 inch pie pan and place first dough sheet on bottom. Fill with layers of the potato mixture. Cover with second dough sheet.
Poke several holes to vent and coat with beaten egg yolk and water wash
Bake at 425 degrees for 10 mintues then reduce heat to 350 degrees cook for 1 hour or until knife inserted finds no resistence.