You need about 3 cups of cleaned leeks cut into 1/8″ or so slices
About ½ teaspoon of salt
2 Tablespoons of good olive oil
2 tablespoons of butter
a pinch of cayenne pepper (peperoncino)
a pinch of nutmeg
about ½ cup milk
salt to taste
100 grams or 3.5 ounces of pecorino fresco not aged
40 to 50 grams of freshly grated Parmigiano Reggiano
When the potatoes are done, drain them and puree them. Puree in the butter and then the eggs.
Add the cayenne and nutmeg, stirring them in. Add the grated pecorino and stir it in.
Heat a wide and heavy frying pan with the olive oil. Add the leeks and salt them. Cook slowly, stirring, until they start to toast and take on a toasted flavor. They do not have to be entirely brown, but browned enough to take on that taste.
When the leeks are done, stir them into the potato mixture. Then stir in enough milk so that the mixture will not pile up, but stays level when you spoon it. Don’t go overboard. Transfer the mixture into a two-quart baking dish, then scatter the Parmigiano over the top. You can at this point cover it tightly with foil and refrigerate it until the next day, but take it out an hour before you want to cook it.
When ready to cook it, put it into a 350°F or 175°C preheated oven and cook for about 45 minutes until puffy and well-browned.
You can use different cheeses, too