Clean the dock, wash it and put in boiling salted water just for it to change color. Take it out, drain and cut up the leaves.
Chop the leeks finely and put to fry, add the dock, fry for 2-3 min. Once cooled, add the eggs and spearmint and stir.
Lay out the phyllo pastry sheets, smear them with the dock, sprinkle feta cheese and a little oil. Wrap into a roll and arrange the roll in a spiral starting from the middle of an oiled tray.
Repeat this process for the rest of the sheets, wrapping the spiral around in one direction. Put pieces of butter on top of the prepared phyllo pastry and bake at 356°F (180 °C) until reddened.