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1
Drain the ricotta in a fine-cloth-lined sieve over a bowl in the refrigerator for 24 hours, so the cold slows any souring while the whey drips away slowly and completely rather than being squeezed out unevenly. The ricotta is ready when it holds its shape when squeezed and leaves the cloth almost dry — a compact, nearly crumbly mass. Press the cloth: if liquid still runs freely, drain further, because any perceptible moisture at the surface will make the final dough slack and impossible to coat cleanly.
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2
Blanch the spinach briefly in boiling salted water, drain, and refresh immediately under cold water so it loses its raw edge without continuing to cook and turn grey. Gather it into a cloth and wring with both hands, twisting progressively tighter, until no further liquid can be extracted — not a single teaspoon of residual moisture may remain, because even a little will loosen the dough and cause the gnudi to tear in the pan. Chop it finely until it resembles a dense, almost fibrous paste.
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3
Combine the drained ricotta, spinach, Parmigiano, eggs, salt, and black pepper in a bowl and mix until the mass is uniform and cohesive. It should hold a soft ball when pressed but not stick aggressively to the hands. If it feels wet or slack, work in an additional tablespoon of Parmigiano, which tightens the mixture without making it dense.
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4
Spread a generous, even layer of fine semolina across a tray, then portion the ricotta mixture into 2-tablespoon rounds and roll each gently between the palms into a smooth ball. Drop each ball immediately into the semolina and roll to coat on all sides thoroughly and evenly, because the semolina draws surface moisture from the ricotta over the resting period, forming a thin, papery skin that is the only thing holding the gnudo together in the water — without it, they dissolve. This is where the dish lives or dies: lay the coated gnudi in a single layer on the semolina-dusted tray, spacing them so they do not touch, and place uncovered in the refrigerator for exactly 24 hours, at the end of which each gnudo should feel lightly firm with a dry, set exterior that does not yield when pressed.
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5
Bring a wide, deep pan of salted water to the gentlest possible simmer — the surface should barely tremble — because a vigorous boil will batter the delicate skin and break the gnudi open before they have a chance to set. Lower the gnudi in batches using a slotted spoon and cook 3–4 minutes. They are done when they float freely to the surface and feel firm, not yielding, when touched with the back of the spoon.
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6
While the gnudi cook, melt the butter in a wide sauté pan over a medium flame and add the sage leaves, so the leaves crisp in the butter as it colours rather than steaming and going limp if added to a cold pan. Cook until the butter turns a deep amber — a smell of toasted hazelnut confirms the noisette stage. Remove from the heat immediately, as the butter continues to cook in the residual heat of the pan.
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7
Transfer the gnudi directly from the water to the pan of brown butter using a slotted spoon, allowing each one a moment to drain so the cooking water does not dilute the butter and turn it greasy. Turn them once with care. Serve immediately, with any remaining brown butter and the crisped sage spooned over — the gnudi should appear smooth-skinned, barely glistening, and hold their round form without flattening on the plate.