An hour before the meal, bring the grated fontina to room temperature and place the sliced black truffles in the milk to infuse it with their flavor.
Make a paste with 300 g (about 10 oz) of the parmesan, the bread crumbs, the lemon zest, the eggs and abundant nutmeg; leave it to rest in refrigerator.
In a double boiler, melt the fontina in the black truffle-infused milk.
Whisk in the egg yolks one at a time, constantly stirring until obtaining smooth and thick. Set aside and keep warm.
Pass the passatelli through the passatelli maker* and gently simmer in hot salted water.
Drain them and toss them into a frying pan with the remaining butter and parmesan.
Spread a pool of the sauce on the warm plates and place the passatelli, then finely shave the white truffle on top.
* or just use a potato masher with fairly large holes, set over the pot of boiling water, and press the mixture through it.