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1
Combine the breadcrumbs, Parmigiano, lemon zest, nutmeg, and salt in a bowl. Add the eggs and, if using, the bone marrow. Work the mixture firmly until it coheres into a smooth, pliable dough. Press a small piece between the fingers — it must hold its shape without crumbling or sticking. If too dry, add no liquid; adjust only with a few extra gratings of Parmigiano. The dough is ready when it resists the thumb cleanly.
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2
Bring the broth to a steady simmer in a wide pan — it should be moving gently but not boiling hard. Press the passatelli dough through the passatelli iron or a potato ricer fitted with 4 mm holes, cutting the cylinders to approximately 4–5 cm with a knife or the edge of the disk, and dropping them directly into the simmering broth in manageable batches. The broth should remain at a simmer throughout.
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3
Cook the passatelli for 2–3 minutes. They are ready when they rise to the surface and feel firm when nudged with a spoon. Remove from the heat immediately — beyond 3 minutes they become waterlogged.
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4
Ladle the passatelli into warmed bowls together with a generous measure of the broth. Serve at once, with additional Parmigiano alongside if desired.