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1
Combine the dry breadcrumbs, grated Parmigiano-Reggiano, nutmeg, and lemon zest in a bowl. Add the eggs and work the mixture with your hands until it coheres into a stiff, compact paste — it should hold its shape when pressed and show no tendency to slump. If the paste is at all slack, it will not hold a cylindrical form after extrusion.
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2
Wrap the paste and rest it in the refrigerator for 30 minutes. It will firm slightly and extrude more cleanly through the disk.
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3
Bring the broth to a rapid, rolling boil. This is where the dish lives or dies: the broth must be at a full boil when the passatelli enter it, so that their exterior sets on contact; a lower temperature causes them to disintegrate before they can hold their shape.
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4
Hold the passatello press or large-holed food mill directly over the pot. Press the paste through the iron disk so that ridged cylinders fall straight into the boiling broth — do not shape on a board and transfer, as the cylinders adhere immediately to any surface. The cylinders should measure 3–5 cm; snap them free of the press with a knife or the back of a spatula.
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5
Cook for 2–3 minutes only. The passatelli are ready when they have firmed on the outside, are tender but coherent throughout, and float near the surface of the broth. At 4 minutes they begin to collapse; remove them at the first sign of softening past tenderness.
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6
Ladle immediately into warmed deep bowls, distributing the passatelli evenly and covering them generously with very hot broth. Serve at once — the eating window is 3 minutes from the bowl reaching the table.