Spinach and Potato Gratin, Izmir Style

A sfongo is a gratin of vegetables thickened with potatoes, eggs, and cheese. This sfongo is unusual in appearance, as additional mashed potato is formed into little mounds that are interspersed in the spinach-and-potato base. You could also make this with Swiss chard, for a sfongo di pasi. In Sefarad Yemekleri, the spinach is chopped and added uncooked. I find that it is easier to mix in the spinach after it has been wilted. Other versions of this recipe omit th