Place the breadcrumbs on a pastry board, form a well in the center, and break the 3 eggs in the center. Add the grated Parmigiano-Reggiano cheese, a grating of nutmeg, the lemon zest, and salt and pepper to taste. A large bowl can also be used as an alternative to the pastry board.
Add a ladle of broth and mix the ingredients with your hands until you get a compact dough. Adjust the breadcrumbs and broth to obtain a compact mixture.
Leave the dough to rest for about 2-3 hours at room temperature. Every 20-25 minutes, knead the dough again for a couple of minutes. This process will help make dough elastic and firm and to prevent the passatelli from disintegrating in the broth.
When the dough is ready, bring the broth to boil.
On a lightly floured work surface, divide the dough into balls big enough to fit in the potato ricer and crush them one by one with a potato masher with large holes (about 5 mm). Wrinkled and firm filaments should form as the dough is forced through the holes.
Slowly and carefully drop the passatelli into the boiling broth.
The passatelli are cooked as soon as they float to the surface.
Traditionally, the soup is made with meat broth, but vegetable or fish broth may also be used.