Place potato quarters in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 15-20 minutes or tender, adding leeks during the last 3 minutes. Drain.
Transfer potato quarters to a large bowl; mash with eggs, bread crumbs, cheese, salt and pepper. Cover and refrigerate for 1 hour.
Heat 1 tablespoon oil in a large cast-iron or other heavy skillet over medium heat. Drop batter by 1/4 cupfuls into oil. Fry in batches until golden brown on both sides, using remaining oil as needed. Drain on paper towels. Serve with sour cream.