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1
Combine the Parmigiano Reggiano, fine breadcrumbs, eggs, flour, nutmeg, and lemon zest in a bowl. Work the mixture with a fork, then the hands, until a coherent, firm dough forms. Press the dough into a ball: it must hold its shape cleanly without sticking to the palm. If it slackens, the passatelli will dissolve in the broth. Cover and rest at room temperature for a minimum of 30 minutes. The dough will firm further as the breadcrumbs absorb the egg.
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2
Bring the broth to a rolling simmer in a wide pan. The broth must be fully simmering — not merely steaming — before the passatelli are pressed. This is where the dish lives or dies: insufficient heat causes the threads to lose their structure and dissolve into the broth rather than cooking firm. Hold the passatelli iron over the broth, fill it with dough, and press steadily so threads of 5–7 mm diameter fall directly into the liquid. Cut each set of threads at about 4–5 cm with a knife.
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3
Cook the passatelli in the simmering broth for 90 seconds only. They are ready when they rise and float freely at the surface, yielding but with clear structure when pressed lightly with a spoon — not slack, not dissolving at the edges.
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4
Ladle immediately into pre-warmed bowls, spooning ample broth over the passatelli. Serve at once. The passatelli continue to absorb broth on standing and will turn mushy within minutes.