Place potatoes in a pot and cover with water. Cover pot and bring to a boil; salt water and cook potatoes until tender, 12-15 minutes. Drain and return potatoes to hot pot.
Meanwhile, heat EVOO over medium heat and melt butter into warm oil. When butter foams, add leeks and wilt 5 minutes, season with salt and pepper.
Mash potatoes with leeks, lemon zest, soft cheese, stock or milk. Season with salt and pepper to taste.