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1
Combine the breadcrumbs and Parmigiano Reggiano in a bowl. Add the beaten eggs, lemon zest, nutmeg, and flour. Work the mixture with your hands until it comes together into a firm, compact dough. It should hold its shape when pressed and not stick to the palm. Cover the bowl and rest at room temperature for 30 minutes — the breadcrumbs must fully absorb the egg; the dough is ready when it no longer feels granular but yields as a single cohesive mass.
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2
Bring the broth to an active simmer in a wide pot — it must be fully simmering, not merely warm. This is where the dish lives or dies: press portions of the dough through the ferro per passatelli or the largest disc of a potato ricer, holding the press directly over the surface of the broth. Cut the emerging worms with a knife or the back of the press at 5–7 cm lengths, allowing them to drop straight into the liquid. They must make contact with actively simmering broth immediately on exiting the press; if the broth is too cool they will fuse together and never separate.
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3
Cook the passatelli in the simmering broth without stirring. They are done when they rise to the surface and have remained floating there for 1 minute — no more than 2–3 minutes total from the moment they float. Overcooking beyond this point causes them to toughen and disintegrate.
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4
Ladle immediately into warmed bowls, distributing the passatelli evenly and covering with the clear broth. Serve at once — the passatelli stale and toughen within 10 minutes in the broth.