Preheat oven to 400 F. Generously oil the bottom of an 8-9 inch cake pan or cast iron skillet.
Place mashed potatoes in large mixing bowl.
Mix butter, feta, and herbs into the mashed potatoes.
In a separate bowl, sift together the flour, salt, and baking powder.
Add the flour mixture to the potatoes, working them together gradually, and adding just enough milk to bind the mixture together into a workable dough (not sticky).
Turn the dough out onto a lightly floured board and knead very lightly until pliable.
Roll into a ball, flatten and roll out into a circle about 8 inches in diameter. Cut into eight wedges.
Transfer the wedges to the oiled pan.
Prepare egg wash - lightly beat one egg with about 1 Tbsp of water.
Brush the top of each scone with egg wash.
Bake until golden brown, about 38 minutes. (Note, these may seem undercooked in the center even when they are done - that's the melty cheese and hot potatoes.)
Remove from the pan immediately! Cast iron needs to be cleaned right away while it’s hot.
Notes
Optional - use 3/4 tsp mixed dried Italian herbs instead of the fresh herbs.