Boil chopped potato ten minutes until tender. Drain.
Boil a saucepan of water. Place fresh spinach in a colander and pour water overtop. Let cool until you can work with it, and then squeeze out all the liquid you can. It works well to let it rest for a minute and then squeeze it again (and again).
In a medium bowl, mix spinach, potatoes, half and half, and 2/3 cup cheese. Add nutmeg, salt and pepper.
Roll pieces of your pie dough into golf-sized balls. Roll out into circles on a floured surface. Place about 1/4 cup of spinach-cheese mixture in the center and fold up. Seal edges by pressing them together with your fingers. Transfer to cookie sheet. Brush crust with egg and top with some of the remaining cheddar and a sprinkling of coarse salt. Repeat with remaining dough and filling.