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1
Line a sieve with cheesecloth and set it over a bowl, then turn the ricotta into it and refrigerate for 12 hours, so that gravity draws away the whey slowly and evenly rather than leaving wet pockets throughout the curd. At the end of the drain, press lightly — the ricotta should feel firm and hold a shape when pressed. If it still weeps liquid, drain it further, because a wet ricotta cannot be corrected once the dough is mixed.
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2
Blanch the spinach in well-salted boiling water for 90 seconds, then drain and refresh in cold water so the leaves stop cooking immediately and keep their bright colour. Wring in a clean cloth until no further moisture can be extracted, then chop finely — press again if in doubt, because any residual water will migrate directly into the ricotta and slacken the mixture beyond recovery.
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3
In a bowl, combine the drained ricotta, chopped spinach, grated Parmigiano, egg yolks, nutmeg, and salt, and work together until uniform. The mixture should be dense enough to hold a rolled ball without slumping. This is where the dish lives or dies: if the mixture appears too slack — soft enough to spread rather than hold — add a further egg yolk and a small amount of semolina before proceeding, then test one gnudo in simmering water and observe whether it holds its shape.
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4
Spread a generous layer of fine semolina across a deep tray, roll the ricotta mixture into balls of approximately 30 g each, and roll each ball immediately in semolina until fully coated, so the semolina begins drawing surface moisture from the gnudo the moment it makes contact. Nestle each ball into the tray and cover completely with further semolina so that every surface contacts it, with no part of the gnudo exposed to air. Place the tray, uncovered, in the refrigerator for 24 hours — at the end of the rest, the surface should feel dry and faintly firm to the touch, telling you that a thin, cohesive skin has formed.
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5
Bring a wide, deep pan of well-salted water to a bare simmer — surface trembling, no rolling boil — because vigorous movement will dissolve the semolina skin before it has time to set in the heat. Gently lift each gnudo from the semolina, shake off the excess lightly, and lower into the water in batches without crowding. The gnudi are ready when they float to the surface and then 45 seconds more — no longer, as residual heat continues the cooking — and should be removed with a slotted spoon.
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6
While the gnudi poach, melt the butter in a wide sauté pan over medium heat and continue cooking until it turns golden amber and smells of toasted hazelnuts, at which point the milk solids have coloured and the flavour has deepened enough to carry the delicate gnudi rather than disappear beneath them. Add the sage leaves and allow them to crisp for 30 seconds — they should be still and glassy at the edges. Remove from heat.
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7
Transfer the poached gnudi directly into the brown butter pan and turn once to coat. Plate immediately, finish with grated Parmigiano, and serve without delay, because the semolina skin softens rapidly once the gnudi leave the heat.